<p>Let me use an analogy as an example of being able to say something about a thing we never see.</p>
<p>I can fairly confidently say that <code>Zinn</code> exist. Why can I say that? I've never met this person (or organisation, or AI) called <code>Zinn</code>! I can say that <code>Zinn</code> exist because he/she/it asked this question that I'm answering to.</p>
<p>I have no idea what exactly is <code>Zinn</code>. I call him/her/it <code>Zinn</code> because that is the self-described name in the user profile. But if this question was asked by an anonymous user I have no name for such an agent. Thus when discussing this question with other Stackexchange users I may need to constantly refer to the user as <code>The anonymous user that asked about why it is OK to say dark matter make up X amount of matter</code> which would be kind of annoying to say that every time. So I may just give such an anonymous user the &quot;nickname&quot; of <strong>&quot;dark user&quot;</strong>.</p>
<p>I can make many confident statements about dark user. Such as their question has two paragraphs. Or their question contains a link. You may ask how can I make such assertion when I've never seen nor met this dark user? Well, because I can see the question on Stackexchange.</p>
<p>Similarly we've measured that there is something affecting the gravitational behavior of everything else in the Universe. We can't see what it is but it is making everything else (galaxies) behave in a way contrary to our math. And we can calculate the weight of this unseen thing thus the mass of it.</p>
<p>We <strong>can</strong>, if we want, call it &quot;the thing that we cannot see but has a gravitational effect on everything else&quot; but saying &quot;the thing that we cannot see but has a gravitational effect on everything else&quot; every time we want to discuss it is annoying. So we gave it a name: dark matter.</p>
<p>Just because we cannot see it does not mean we can't see the effect it has on the universe.</p>

CC BY-SA 4.0
2024-03-21T06:21:01.623Z
2024-03-21T13:08:41.607Z
2024-03-21T13:08:41.607Z
slebetman
<p>Ultimately, the presence of invisible mass is the most straightforward explanation for what we're seeing. We're aware that we could be wrong, but if we are, we're wrong about some of the fundamentals of physics.</p>
<p>We can see that there is an effect, and the only thing <em>we know of</em> that could cause that effect is mass, which is a property of matter. <em>Something</em> is changing the orbits of visible objects and bending light from behind it, among other things. Based on the influences we can observe, we can estimate how much mass is there and how it's distributed, and when we do that, the numbers suggest that there's a lot more mass that is &quot;dark&quot; -- not glowing and not blocking light to any significant degree -- than mass that we can see as &quot;normal&quot; stuff like stars and dust clouds.</p>
<p>I suppose it's <em>possible</em> there isn't any invisible, intangible matter out there. Maybe we'll figure out that there's some trick to how gravity works on huge scales that explains the discrepancies without requiring more than 3/4ths of the matter in the universe to be &quot;dark&quot;. Maybe there's more than one explanation and what we call &quot;dark matter&quot; today is actually half a dozen distinct effects that we've incorrectly collected into one category. We'll somehow have to explain things like galactic collisions that appear to have separated the light and dark matter into two distinct blobs, both bending light but only one visible to our instruments, but -- sure. It's possible there's some way to handle it that doesn't require dark matter.</p>
<p>However, until we have a good reason to think that we aren't observing mundane gravitational effects from matter we can't detect, we're going to talk about it that way.</p>
<p>That's kinda how science works -- we often talk about the best current explanation as if it's true, while we all understand that it could potentially be overturned tomorrow. And why not? That's the state of all knowledge. <em>Anything</em> we know is subject to being replaced if somebody comes up with a new explanation that fits all the observations better than the old explanation and stands up to extensive scrutiny.</p>

CC BY-SA 4.0
2024-03-20T18:00:52.350Z
I agree with most of this, but it gives the impression that dynamics and lensing are the reasons the matter must be "dark". Normal matter (cold, condensed baryons - white dwarfs, planets, black holes etc.) could produce those effects but (a) we can't find any and (b) normal matter like that cannot explain the primordial abundances of He and D, the fluctuations in the CMB or sturucture formation in the universe - and neither can MOND (afaik).
ProfRob
CC BY-SA 4.0
2024-03-21T16:40:42.183Z
Is this an application of [Occam's razor](https://en.wikipedia.org/wiki/Occam%27s_razor)?
gidds
CC BY-SA 4.0
2024-03-22T06:47:36.157Z
@Gidds: Yes it is. Doctors in the 1600s are trying to understand plague without knowing that bacteria exist. Occam's razor yields the least improbable outrageous claims involving invisible matter and impossible physics because they can't say "we don't know why this happens." Novel models should be rejected for phenomena we understand, but encouraged in the "dark energy" matter. A similar thing is going on in physical anthropology (my first college major, so I follow it).
Miss Understands
CC BY-SA 4.0
2024-03-23T00:47:01.133Z
Thanks for your response @Darth Pseudonym. I think your last paragraph really gets to the heart of what prompted me to write the question. I have come across headlines in recent years suggesting that a modified gravity theory has had a noteworthy breakthrough. I'm not on a bandwagon, but the fact that there is still this level of uncertainty suggests to me that we could be doing harm by talking about dark matter like we understand it more that we do. It might be ok for the old heads, but I think students could benefit from being made aware of our ignorance more clearly.
Zinn
CC BY-SA 4.0
2024-03-23T05:11:41.340Z
@Zinn I think this often comes up in "informal" discussions of science, especially of cutting-edge science. (Caveat/disclaimer: I'm not a scientist, but I do read pop-sci type stuff.) The problem is that if you properly qualified every statement, everything you would say would be majority qualification and a bit of actual substance sprinkled in there.
yshavit
CC BY-SA 4.0
2024-03-23T05:14:11.643Z
For example, we know according to relativity that time moves slower as you move faster. But actually: "our current, modern understanding of relativity, which may be demonstrated to be inaccurate in the future — and it hasn't been unified with quantum mechanics, so it's possible there's a big unknown that could throw everything out the window tomorrow — suggests that it is very likely that time moves slower as you move faster." It's very hard to have a conversation when so much of it is filler that can be just implicitly understood.
yshavit
CC BY-SA 4.0
2024-03-20T16:27:45.587Z
2024-03-21T14:00:55.210Z
2024-03-21T14:00:55.210Z
Darth Pseudonym
<p>We've extensively studied dark matter properties using the effects of its gravity. It's no different than the Sun, really. We've only know what the Sun is made of and where it gets its energy from for about a century, but we've known of its existence from the effects of that mass and energy for as long as there have been people.</p>

CC BY-SA 4.0
2024-03-20T20:30:44.730Z
[Cecilia Payne-Gaposchkin](https://en.wikipedia.org/wiki/Cecilia_Payne-Gaposchkin) "proposed in her 1925 doctoral thesis that stars were composed primarily of hydrogen and helium. Her groundbreaking conclusion was initially rejected because it contradicted the scientific wisdom of the time, which held that there were no significant elemental differences between the Sun and Earth".
PM 2Ring
CC BY-SA 4.0
2024-03-22T06:58:04.743Z
@pm: yes, but she was a woman in 1925. Einstein had to intercede for M. Curie, who's contributions far exceeded Cece's. And until 1960, female astronomers couldn't book time at Jodrell bank because the facility didn't have a ladies' room.
Miss Understands
CC BY-SA 4.0
2024-03-22T08:00:26.907Z
@Miss_Understands Indeed! As Wikipedia says, Henry Norris Russell initially convinced Cecilia to tone down her conclusions, and when he changed his mind 4 years later, mentioning her work, he was generally credited for the conclusions she reached. :(
PM 2Ring
CC BY-SA 4.0
2024-03-22T08:03:03.873Z
Here are some other women who made important contributions to astronomy, mostly underrated. Sophia Brahe, Émilie du Châtelet, Caroline Herschel, Annie Jump Cannon, and Henrietta Swan Leavitt. In more modern times we have Margaret Burbidge, Jocelyn Bell Burnell, Michelle Chapront-Touzé, and Nicole Capitaine, but they've still had to work against gender bias. Also see *Pythagoras' Trousers* by Margaret Wertheim.
PM 2Ring
CC BY-SA 4.0
2024-03-22T12:36:29.260Z
And the woman who explained mass loss after nuclear reactions, and the pulsar lady, and Hypathia of Alexandria.
Miss Understands
CC BY-SA 4.0
2024-03-20T14:14:31.117Z
2024-03-20T14:14:31.117Z
John Doty
<p>This is a slightly more technical answer that should nonetheless illustrate the concepts.</p>
<p>Wind back the clock to 1845. You are an astronomer of that period, and you are familiar with Newton's Laws, which have been extensively verified to work in the lab. You observe Uranus and notice that it moves largely as Newton's Laws predict it to, but there is some disparity with the theoretical prediction. What could be causing the discrepancy? The two obvious explanations are:*</p>
<ol>
<li>Newton's Laws aren't actually right. They work in the lab, but once you extend to 2.9 billion kilometers (the average distance from the Uranus to the Sun), it no longer works.</li>
<li>There's something else &quot;out there&quot; that is exerting extra gravity and causing the discrepancy.</li>
</ol>
<p>With option 2, you can go further: if you assume that Newton's Laws are right, then you can calculate more properties of the extra something: its mass, its orbit, and so on. But note Newton's Laws only say the how much mass the new body has. It doesn't say what the new body is. It could be a new planet, or it could be something else. This is how Neptune was discovered - it was a theoretical prediction based on Newton's Laws.</p>
<p>The situation with dark matter is similar. If you assume General Relativity is correct, then there's a discrepancy between theory and observations that can be resolved by postulating the existence of dark matter. You don't know what dark matter is, but you can calculate how much dark matter there should be (which is where the 85% came from).</p>
<p>Or you could say General Relativity is incorrect, which might also invalidate the need for dark matter. This also happened historically - Mercury shows precession that Newton's laws could not explain, leading some to conjecture that there must be another planet closer to the Sun than Mercury. The missing planet (Vulcan) was never found, and General Relativity removed the need for the planet. In the same way, if MOND turns out to be correct, then dark matter would be unnecessary, and the statement that dark matter makes up 85% of the matter content would be incorrect.</p>
<p>At present, most astrophysicists believe dark matter is strongly favored over alternative explanations, which is why they are happy to make the statements you allude to. The preponderance of evidence is in favor of dark matter** - but they might all get egg on their faces if alternative gravity turns out to be right.</p>
<hr />
<p>*Of course, in practice, it's significantly more complicated. Confounding factors could be: perhaps Uranus's mass was estimated incorrectly, or the distance to Uranus was estimated incorrectly, or your observations of Uranus are incorrect because of some atmospheric fluctuation in the Earth, etc.</p>
<p>**I suspect what really convinces most astrophysicists is the <a href="https://en.wikipedia.org/wiki/Dark_matter#Observational_evidence" rel="nofollow noreferrer">sheer number of independent lines of evidence</a> that can all be explained by standard cold dark matter. An alternative hypothesis might be able to explain any individual line of evidence, but explaining <em>all</em> of them with one framework is extremely difficult. No alternative theory has come close, but as long as dark matter is not identified, some people will continue to think about them.</p>

CC BY-SA 4.0
2024-03-28T09:44:29.727Z
@Sten can you point out where in the paper it says there is still dark matter at early times? I skimmed the paper but did not see it - the closest is on page 2 when it says there must be sizable amounts of energy density scaling as $a^{-3}$, but the first sentence of the next paragraph also says that there are scalar fields that can mimic that dependence.
Allure
CC BY-SA 4.0
2024-03-28T21:11:23.090Z
@Allure For example: "For a wide range of parameters, this relativistic MOND theory is consistent with the CMB measurements from Planck. This happens because $c^2_\mathrm{ad}$ and $w$ are small enough so that $\Pi\to 0$ and we get dustlike evolution . . ." Basically they have a field that behaves as dust (i.e. dark matter) and presumably has the same abundance, although the PRL is extremely light on details and I cannot find the promised ("in preparation") ref. [104]. They do note that the abundance "is not (classically) predicted" so I assume the data will force it to the known DM abundance.
Sten
CC BY-SA 4.0
2024-03-21T07:13:24.223Z
But keep in mind MOND is only a substitute for some of the predictions of dark matter. It's not really an alternative to dark matter; it's an alternative to *the effect of dark matter on galactic dynamics*.
Sten
CC BY-SA 4.0
2024-03-21T07:44:58.633Z
You could extend this analogy. Newtonian mechanics is unable to explain the orbit of Mercury. Astronomers searched for an unseen inner planet called Vulcan, couldn't find it and eventually GR superseded Newtonian theory. i.e. It works both ways and it still might in the case of dark matter (except there is more variety of evidence for dark matter than there ever was for Vulcan).
ProfRob
CC BY-SA 4.0
2024-03-21T08:05:19.033Z
@ProfRob good point, I've made the change.
Allure
CC BY-SA 4.0
2024-03-21T08:07:19.153Z
@Sten as far as I'm aware, there are efforts to get MOND (or its relativistic generalizations) to match cosmological data, e.g. https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.127.161302
Allure
CC BY-SA 4.0
2024-03-21T16:10:11.830Z
Note that theory still has dark matter at early times and basically suppresses its mass in time for galaxies.
Sten
CC BY-SA 4.0
2024-03-21T21:32:04.313Z
@ProfRob I've wondered recently about how confidently the existence of Vulcan was asserted prior to GR.  Did the inability to find Vulcan cause doubt or was it more like the way the aether was confidently asserted?  The inability to find/identify dark matter doesn't seem to generate much doubt about its existence.
JimmyJames
CC BY-SA 4.0
2024-03-21T21:53:01.050Z
@JimmyJames that is because there is a wide variety of evidence (not just dynamics) for dark matter. However, on what basis do you assert that there isn't much doubt? That seems to be myth propagated by those who wish to appear contrarian. No proper scientist I know thinks dark matter is certain.
ProfRob
CC BY-SA 4.0
2024-03-21T21:55:41.363Z
@ProfRob It's the vehement dismissiveness with which people push back on any question of its existence.  Look at the discussion here for examples.
JimmyJames
CC BY-SA 4.0
2024-03-22T00:36:57.293Z
I would say dark matter in the early universe is about as certain as anything in science can be. (There's a reason MOND still needs it there!) Late universe I think the point we'll be able to say it's confirmed (assuming no nongravitational detection) is when we find clear evidence of the gravity of a subhalo with no stars/gas. (Disclaimer: I work in dark matter.)
Sten
CC BY-SA 4.0
2024-03-22T13:49:12.910Z
Damn, that's an elegant answer, Allure. You should write science documentaries. I want to vote it up 3 times.
Miss Understands
CC BY-SA 4.0
2024-03-21T07:08:26.073Z
2024-03-28T09:52:44.330Z
2024-03-28T09:52:44.330Z
Allure
<p>The presence of a third family of <a href="https://en.wikipedia.org/wiki/Top_quark" rel="noreferrer">massive quarks</a> was theoretically predicted in 1973, based on clues provided by other particle physics experiments.</p>
<p>The bottom quark was discovered in 1975, but the top quark was not revealed until 1995.</p>
<p>I think this is a reasonably apt analogy. There are plenty of observational clues that something called dark matter exists. These include: the dynamics of stars and gas in galaxies and the dynamics of galaxies in clusters; the gravitational lensing of light by clusters; the temperature fluctuations in the cosmic microwave background; the primordial abundance of helium and deuterium; and the way that cosmic structure has developed as the universe has evolved.</p>
<p>Attempts to work out what this dark matter is have proceeded in a scientific fashion - ruling out possibilities one-by-one whilst also making attempts at direct detection. No <em>proper scientist</em> ever makes the claim that dark matter <em>must</em> exist, and there are plenty of scientists out there suggesting and pursuing alternatives. Most, if not all, of these alternatives offer a less comprehensive explanation of observations than the unpalatable hypothesis that we do not yet understand what 80+% of the gravitational matter in the universe is.</p>

CC BY-SA 4.0
2024-03-21T07:09:56.827Z
2024-03-21T07:17:17.227Z
2024-03-21T07:17:17.227Z
ProfRob
<p>Your question is a &quot;wag the dog&quot; one. Observations indicate that there is mass which is otherwise unaccounted for (and more specifically, 85% of the mass present in the universe). This is, then, by definition, what &quot;dark matter&quot; refers to.</p>
<p>Several of the other answers here already make this point, more or less, but I feel compelled to post an answer which points this out succinctly.</p>

CC BY-SA 4.0
2024-03-23T01:10:22.007Z
I was under the impression that "dark matter" should be thought of as placeholder for a future theory that can explain a set of phenomena that we're currently confused about. 

Stating explicitly that 85% of the universe is made up of dark matter seems to be jumping the gun.
Zinn
CC BY-SA 4.0
2024-03-23T01:37:05.350Z
@Zinn: No, placeholder is a fair term, but it’s not jumping the gun.  In fact it’s just opposite: we don’t need the place holder for 15% of the matter in the universe.
jmoreno
CC BY-SA 4.0
2024-03-23T02:03:14.403Z
@Zinn To the extent that "dark matter" might be regarded as a placeholder term, it's a placeholder *for a more precise picture of what the matter is.* Not a placeholder for some alternative explanation that doesn't involve matter.
Sten
CC BY-SA 4.0
2024-03-22T07:24:37.470Z
2024-03-22T07:24:37.470Z
tripleee
<p>Your question is actually spot on, and I disagree with the other answers here. There is indeed a contradiction. But the problem is not with the statement about the percentage of dark matter to matter, the problem is with the statement &quot;We don't know what dark matter is&quot;.</p>
<p>We do. We do know what it is. It is that which makes up 85% of the matter in the universe.</p>
<p>The statement &quot;We don't know what X <em>is</em>&quot; is used FAR too much in science. It is a philosophical statement which can never be fully answered to exhaustion. We don't know what <em>electrons</em> are, not really. Are they fundamental point particles, emergent properties of a field, just a representation of an underlying mathematical symmetry, all of the above? We don't know what <em>viruses</em> are, are they nonliving, are they another branch of life, a different form of life entirely? We don't know even know what <em>HUMANS</em> are. Are neanderthals considered human, is a fetus a human, are genetically modified humans human? You can see how the question is essentially philosophical.</p>
<p>There is a lot we know about dark matter, there is a lot we don't. To say we don't know what it is simply because we are unsure of the exact subatomic makeup or relation to gravity, when we very well knew what things were for hundreds of years before we knew what gravity or an atom even was, is just silly.</p>
<p>There is indeed a contradiction.</p>

CC BY-SA 4.0
2024-03-26T19:44:15.853Z
@Zinn Possibility number (3) has pretty much been ruled out by both theory and observation, it really is a small, vocal minority which is still pushing various modifications of MOND as an explanation. It is one of those theories, like string theory, that can forever be modified with more and more parameters to continually make up for its inconsistency with what is observed.
Anthony Khodanian
CC BY-SA 4.0
2024-03-26T19:48:55.333Z
@ScienceSnake That epistemological problem is exactly what my answer is attempting to avoid. Too often people claim something is unknown because it has not been exhaustively proven, but the thing is nothing can be. It is not presumptious to call something known with only partial knowledge, because nothing is known with *absolute* certainty. Sure, we know a lot more about electrons than dark matter, but we do actually know a heck of a lot about dark matter. We have gigabytes of astronomical data back it up, to put the bar at dark matter would exclude so much of the mundane we know far less of.
Anthony Khodanian
CC BY-SA 4.0
2024-03-21T19:16:02.427Z
Totally agree with this. We know, to a large extent, how dark matter is described in terms of classical continuum mechanics. What we don't know is the *particle-level* description of dark matter. Dark matter is not even unique in that way: we don't know the particle-level description of gravity either, and does anyone object to talking about the effects of gravity?
Sten
CC BY-SA 4.0
2024-03-22T06:57:08.660Z
We don't know whether it is (1) lots of massive particles, (2) far fewer primordial black holes, (3) not there at all. To me that is pretty much as clear a "we don't know what it is" as there could be.
ProfRob
CC BY-SA 4.0
2024-03-23T01:16:58.313Z
@ProfRob it's the possibility of number (3) which makes the statement "dark matter makes up about 85% of the cosmos" seem presumptuous to me
Zinn
CC BY-SA 4.0
2024-03-21T19:06:55.110Z
2024-03-21T19:06:55.110Z
Anthony Khodanian
<p>Well, first off, given how contradictory recent studies have shown to be, MOND isn't exactly a theory that's backed up by much anymore. It is in kind of a foggy place at the moment. It could still be a possibility, as you have stated ; but there would be a lot of issues remaining before being able to consider it to its full extent.</p>
<p>Now as for just how one may be able to derive such a proportion despite their absolute ignorance regarding what this supposed (<em>supposed !</em>) dark matter exactly might be, or exactly how it should behave in the grand scheme of things ; you must remember that this is purely a theory, and at that, one that was derived from observation --as to fit it.</p>
<p>An offset between reality as we observe it to be and that which we compute through models based on relativity is present at most scales, and increases the greater the scale.
In order to &quot;fix&quot; these discrepancies, <em>some of</em> the leading theories on the matter (pun intended) derived the existence of some kind of &quot;other&quot; matter --that had mass, for certain, as that is how it would come to solve the issue in the first place, but which had to interact differently with &quot;usual&quot; (or regular) mass ; hence our inability to quite simply observe it.</p>
<p>Knowing this, it becomes obvious where this proportion arose from ; as this supposed &quot;dark matter&quot; is used as a kind of fix for models involving relativity to coincide with reality, a certain amount of it (which varies depending on the exact theory you consider to be the closest match) and only that amount would &quot;solve&quot; the issue.</p>
<p>Looking at the way elements of a galaxy spin around its center black hole for instance, it becomes apparent that either the model is missing something (hidden variables at a macroscopic scale), or that there is more to what we are currently able to observe.
The latter option is what dark matter variations aim to solve, through a type of matter that only loosely interacts with other kinds of matter.</p>
<hr />
<p>Perhaps this could even be attributed to vacuum energy and relatively empty space's proximity to massive celestial bodies, stretching apart this energy of emptiness to different extents. (take this with a grain of salt, that last bit isn't backed up by anything as it is but a thought that has occurred to me which I haven't got to look into just yet)</p>

CC BY-SA 4.0
2024-03-21T22:14:31.077Z
2024-03-21T22:14:31.077Z
MaxAnimator0817
<p>It strikes me as contradictory that the scientific community will say that we don't know what dark matter is, but be happy to state things like &quot;dark matter makes up about 85% of the cosmos&quot; (source: <a href="https://phys.org/news/2024-03-astrophysicist-hint-dark.html" rel="noreferrer">phys.org</a>)</p>
<p>Is there something wrong with the way I'm thinking about this? If <a href="https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics" rel="noreferrer">MOND</a> is correct for example, which seems to be a possibility still, wouldn't this mean that the statement about dark matter making up a certain percentage of the universe be false?</p>

CC BY-SA 4.0
2024-03-20T15:36:04.997Z
I suppose we could try to require everyone to add five more paragraphs covering all (well, not all) possible alternate explanations for the observations. Or realize that there are other theories, but if the universe does obey current accepted ones there is 85% of dark mass out there that we don't know what it is.
Jon Custer
CC BY-SA 4.0
2024-03-20T22:20:56.763Z
Yes, statements like this are predicted on the condition "if dark matter is the explanation for the effects we're seeing".  Since the dark matter hypothesis is fairly widely accepted, new stories and statements about refinements to and predictions from that model often omit the "if it's true" part.
Peter Cordes
CC BY-SA 4.0
2024-03-20T22:56:32.123Z
Keep in mind the "85%" figure comes from the cosmic microwave background, which is measured to incredible precision. We are way past "galaxy cluster velocity dispersions are too high" and "galaxies are rotating too quickly".
Sten
CC BY-SA 4.0
2024-03-21T06:50:31.003Z
Imagine that you have two identical boxes. The other box is closed, and the other is open and you can see that it's empty. You then weigh the boxes on a scale and observe that the empty box weighs 0.15 kg and the closed box weighs exactly 1 kg. You might then deduce that the contents of the closed box weigh 0.85 kg, but how is that possible if you don't know anything about what the contents are?
Moyli
CC BY-SA 4.0
2024-03-22T21:46:14.137Z
long before the higgs boson was discovered physicists had a very good idea what it should look like and what properties it would have, because the standard model said such a thing should exist.  of course it could also have been the case that the standard model was incorrect in some way.  that seems rather analogous to how dark matter is conceived.  or how string based theories require particles to exist that have yet to be observed -- could be the case that we don't have the experimental power yet, or that the theory is wrong.
eps
CC BY-SA 4.0
2024-03-22T21:49:29.953Z
so sure, it is possible that gravity simply acts much differently on large scales than small, the lure of a dark matter solution is that it is compatible with relativity, instead of a solution that requires a complete teardown and reworking of gravity models.
eps
CC BY-SA 4.0
2024-03-23T04:05:31.550Z
One of the ways that physics works best is that, based on observation and theory, it makes predictions about new observations: the Higgs boson (as in the comment of @eps); gravitational waves; the positron; and many other predictions are examples. Current observation and theory predicts dark matter, and you can bet physicists would like to observe it... but they haven't yet.
Lee Mosher
CC BY-SA 4.0
2024-03-23T13:09:05.260Z
@Moyli In that analogy the sceptic would say, "But what if the scale is imprecise, or there is a magnet under it that repels the contents of one box and attracts the contents of the other...", etc. The more certain thing is to state that measurements of x and y have been obtained, rather than that the things weigh x and y. (Not a sceptic, just refining the analogy.)
Luke Sawczak
<cosmology><dark-matter>
Why is it ok for people to be saying that dark matter makes up x amount of the universe when we don't know what it is?
Zinn
# Why is it ok for people to be saying that dark matter makes up x amount of the universe when we don't know what it is?

(Asked by: username_0 on 2024-03-20)

It strikes me as contradictory that the scientific community will say that we don't know what dark matter is, but be happy to state things like "dark matter makes up about 85% of the cosmos" (source: [phys.org](https://phys.org/news/2024-03-astrophysicist-hint-dark.html))

Is there something wrong with the way I'm thinking about this? If [MOND](https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics) is correct for example, which seems to be a possibility still, wouldn't this mean that the statement about dark matter making up a certain percentage of the universe be false?

**Tags:** cosmology, dark-matter
> **Comments:**
> **username_1**: I suppose we could try to require everyone to add five more paragraphs covering all (well, not all) possible alternate explanations for the observations. Or realize that there are other theories, but if the universe does obey current accepted ones there is 85% of dark mass out there that we don't know what it is.  
> **username_2**: Yes, statements like this are predicted on the condition "if dark matter is the explanation for the effects we're seeing".  Since the dark matter hypothesis is fairly widely accepted, new stories and statements about refinements to and predictions from that model often omit the "if it's true" part.  
> **username_3**: Keep in mind the "85%" figure comes from the cosmic microwave background, which is measured to incredible precision. We are way past "galaxy cluster velocity dispersions are too high" and "galaxies are rotating too quickly".  
> **username_4**: Imagine that you have two identical boxes. The other box is closed, and the other is open and you can see that it's empty. You then weigh the boxes on a scale and observe that the empty box weighs 0.15 kg and the closed box weighs exactly 1 kg. You might then deduce that the contents of the closed box weigh 0.85 kg, but how is that possible if you don't know anything about what the contents are?  
> **username_5**: long before the higgs boson was discovered physicists had a very good idea what it should look like and what properties it would have, because the standard model said such a thing should exist.  of course it could also have been the case that the standard model was incorrect in some way.  that seems rather analogous to how dark matter is conceived.  or how string based theories require particles to exist that have yet to be observed -- could be the case that we don't have the experimental power yet, or that the theory is wrong.  
> **username_5**: so sure, it is possible that gravity simply acts much differently on large scales than small, the lure of a dark matter solution is that it is compatible with relativity, instead of a solution that requires a complete teardown and reworking of gravity models.  
> **username_6**: One of the ways that physics works best is that, based on observation and theory, it makes predictions about new observations: the Higgs boson (as in the comment of @eps); gravitational waves; the positron; and many other predictions are examples. Current observation and theory predicts dark matter, and you can bet physicists would like to observe it... but they haven't yet.  
> **username_7**: @Moyli In that analogy the sceptic would say, "But what if the scale is imprecise, or there is a magnet under it that repels the contents of one box and attracts the contents of the other...", etc. The more certain thing is to state that measurements of x and y have been obtained, rather than that the things weigh x and y. (Not a sceptic, just refining the analogy.)  

---

## 8 answers

### Answer by username_9 on 2024-03-20. Score: 56

Ultimately, the presence of invisible mass is the most straightforward explanation for what we're seeing. We're aware that we could be wrong, but if we are, we're wrong about some of the fundamentals of physics.

We can see that there is an effect, and the only thing *we know of* that could cause that effect is mass, which is a property of matter. *Something* is changing the orbits of visible objects and bending light from behind it, among other things. Based on the influences we can observe, we can estimate how much mass is there and how it's distributed, and when we do that, the numbers suggest that there's a lot more mass that is "dark" -- not glowing and not blocking light to any significant degree -- than mass that we can see as "normal" stuff like stars and dust clouds.

I suppose it's *possible* there isn't any invisible, intangible matter out there. Maybe we'll figure out that there's some trick to how gravity works on huge scales that explains the discrepancies without requiring more than 3/4ths of the matter in the universe to be "dark". Maybe there's more than one explanation and what we call "dark matter" today is actually half a dozen distinct effects that we've incorrectly collected into one category. We'll somehow have to explain things like galactic collisions that appear to have separated the light and dark matter into two distinct blobs, both bending light but only one visible to our instruments, but -- sure. It's possible there's some way to handle it that doesn't require dark matter.

However, until we have a good reason to think that we aren't observing mundane gravitational effects from matter we can't detect, we're going to talk about it that way.

That's kinda how science works -- we often talk about the best current explanation as if it's true, while we all understand that it could potentially be overturned tomorrow. And why not? That's the state of all knowledge. *Anything* we know is subject to being replaced if somebody comes up with a new explanation that fits all the observations better than the old explanation and stands up to extensive scrutiny.

> **Comments:**
> **username_10**: I agree with most of this, but it gives the impression that dynamics and lensing are the reasons the matter must be "dark". Normal matter (cold, condensed baryons - white dwarfs, planets, black holes etc.) could produce those effects but (a) we can't find any and (b) normal matter like that cannot explain the primordial abundances of He and D, the fluctuations in the CMB or sturucture formation in the universe - and neither can MOND (afaik).  
> **username_11**: Is this an application of [Occam's razor](https://en.wikipedia.org/wiki/Occam%27s_razor)?  
> **username_12**: @Gidds: Yes it is. Doctors in the 1600s are trying to understand plague without knowing that bacteria exist. Occam's razor yields the least improbable outrageous claims involving invisible matter and impossible physics because they can't say "we don't know why this happens." Novel models should be rejected for phenomena we understand, but encouraged in the "dark energy" matter. A similar thing is going on in physical anthropology (my first college major, so I follow it).  
> **username_0**: Thanks for your response @Darth Pseudonym. I think your last paragraph really gets to the heart of what prompted me to write the question. I have come across headlines in recent years suggesting that a modified gravity theory has had a noteworthy breakthrough. I'm not on a bandwagon, but the fact that there is still this level of uncertainty suggests to me that we could be doing harm by talking about dark matter like we understand it more that we do. It might be ok for the old heads, but I think students could benefit from being made aware of our ignorance more clearly.  
> **username_13**: @Zinn I think this often comes up in "informal" discussions of science, especially of cutting-edge science. (Caveat/disclaimer: I'm not a scientist, but I do read pop-sci type stuff.) The problem is that if you properly qualified every statement, everything you would say would be majority qualification and a bit of actual substance sprinkled in there.  
> **username_13**: For example, we know according to relativity that time moves slower as you move faster. But actually: "our current, modern understanding of relativity, which may be demonstrated to be inaccurate in the future — and it hasn't been unified with quantum mechanics, so it's possible there's a big unknown that could throw everything out the window tomorrow — suggests that it is very likely that time moves slower as you move faster." It's very hard to have a conversation when so much of it is filler that can be just implicitly understood.  

---

### Answer by username_8 on 2024-03-21. Score: 26

Let me use an analogy as an example of being able to say something about a thing we never see.

I can fairly confidently say that `Zinn` exist. Why can I say that? I've never met this person (or organisation, or AI) called `Zinn`! I can say that `Zinn` exist because he/she/it asked this question that I'm answering to.

I have no idea what exactly is `Zinn`. I call him/her/it `Zinn` because that is the self-described name in the user profile. But if this question was asked by an anonymous user I have no name for such an agent. Thus when discussing this question with other Stackexchange users I may need to constantly refer to the user as `The anonymous user that asked about why it is OK to say dark matter make up X amount of matter` which would be kind of annoying to say that every time. So I may just give such an anonymous user the "nickname" of **"dark user"**.

I can make many confident statements about dark user. Such as their question has two paragraphs. Or their question contains a link. You may ask how can I make such assertion when I've never seen nor met this dark user? Well, because I can see the question on Stackexchange.

Similarly we've measured that there is something affecting the gravitational behavior of everything else in the Universe. We can't see what it is but it is making everything else (galaxies) behave in a way contrary to our math. And we can calculate the weight of this unseen thing thus the mass of it.

We **can**, if we want, call it "the thing that we cannot see but has a gravitational effect on everything else" but saying "the thing that we cannot see but has a gravitational effect on everything else" every time we want to discuss it is annoying. So we gave it a name: dark matter.

Just because we cannot see it does not mean we can't see the effect it has on the universe.


---

### Answer by username_16 on 2024-03-21. Score: 17

This is a slightly more technical answer that should nonetheless illustrate the concepts.

Wind back the clock to 1845. You are an astronomer of that period, and you are familiar with Newton's Laws, which have been extensively verified to work in the lab. You observe Uranus and notice that it moves largely as Newton's Laws predict it to, but there is some disparity with the theoretical prediction. What could be causing the discrepancy? The two obvious explanations are:\*

1. Newton's Laws aren't actually right. They work in the lab, but once you extend to 2.9 billion kilometers (the average distance from the Uranus to the Sun), it no longer works.
2. There's something else "out there" that is exerting extra gravity and causing the discrepancy.

With option 2, you can go further: if you assume that Newton's Laws are right, then you can calculate more properties of the extra something: its mass, its orbit, and so on. But note Newton's Laws only say the how much mass the new body has. It doesn't say what the new body is. It could be a new planet, or it could be something else. This is how Neptune was discovered - it was a theoretical prediction based on Newton's Laws.

The situation with dark matter is similar. If you assume General Relativity is correct, then there's a discrepancy between theory and observations that can be resolved by postulating the existence of dark matter. You don't know what dark matter is, but you can calculate how much dark matter there should be (which is where the 85% came from).

Or you could say General Relativity is incorrect, which might also invalidate the need for dark matter. This also happened historically - Mercury shows precession that Newton's laws could not explain, leading some to conjecture that there must be another planet closer to the Sun than Mercury. The missing planet (Vulcan) was never found, and General Relativity removed the need for the planet. In the same way, if MOND turns out to be correct, then dark matter would be unnecessary, and the statement that dark matter makes up 85% of the matter content would be incorrect.

At present, most astrophysicists believe dark matter is strongly favored over alternative explanations, which is why they are happy to make the statements you allude to. The preponderance of evidence is in favor of dark matter\*\* - but they might all get egg on their faces if alternative gravity turns out to be right.

---

\*Of course, in practice, it's significantly more complicated. Confounding factors could be: perhaps Uranus's mass was estimated incorrectly, or the distance to Uranus was estimated incorrectly, or your observations of Uranus are incorrect because of some atmospheric fluctuation in the Earth, etc.

\*\*I suspect what really convinces most astrophysicists is the [sheer number of independent lines of evidence](https://en.wikipedia.org/wiki/Dark_matter#Observational_evidence) that can all be explained by standard cold dark matter. An alternative hypothesis might be able to explain any individual line of evidence, but explaining *all* of them with one framework is extremely difficult. No alternative theory has come close, but as long as dark matter is not identified, some people will continue to think about them.

> **Comments:**
> **username_3**: But keep in mind MOND is only a substitute for some of the predictions of dark matter. It's not really an alternative to dark matter; it's an alternative to *the effect of dark matter on galactic dynamics*.  
> **username_10**: You could extend this analogy. Newtonian mechanics is unable to explain the orbit of Mercury. Astronomers searched for an unseen inner planet called Vulcan, couldn't find it and eventually GR superseded Newtonian theory. i.e. It works both ways and it still might in the case of dark matter (except there is more variety of evidence for dark matter than there ever was for Vulcan).  
> **username_16**: @ProfRob good point, I've made the change.  
> **username_16**: @Sten as far as I'm aware, there are efforts to get MOND (or its relativistic generalizations) to match cosmological data, e.g. https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.127.161302  
> **username_3**: Note that theory still has dark matter at early times and basically suppresses its mass in time for galaxies.  
> **username_17**: @ProfRob I've wondered recently about how confidently the existence of Vulcan was asserted prior to GR.  Did the inability to find Vulcan cause doubt or was it more like the way the aether was confidently asserted?  The inability to find/identify dark matter doesn't seem to generate much doubt about its existence.  
> **username_10**: @JimmyJames that is because there is a wide variety of evidence (not just dynamics) for dark matter. However, on what basis do you assert that there isn't much doubt? That seems to be myth propagated by those who wish to appear contrarian. No proper scientist I know thinks dark matter is certain.  
> **username_17**: @ProfRob It's the vehement dismissiveness with which people push back on any question of its existence.  Look at the discussion here for examples.  
> **username_3**: I would say dark matter in the early universe is about as certain as anything in science can be. (There's a reason MOND still needs it there!) Late universe I think the point we'll be able to say it's confirmed (assuming no nongravitational detection) is when we find clear evidence of the gravity of a subhalo with no stars/gas. (Disclaimer: I work in dark matter.)  
> **username_12**: Damn, that's an elegant answer, Allure. You should write science documentaries. I want to vote it up 3 times.  
> **username_16**: @Sten can you point out where in the paper it says there is still dark matter at early times? I skimmed the paper but did not see it - the closest is on page 2 when it says there must be sizable amounts of energy density scaling as $a^{-3}$, but the first sentence of the next paragraph also says that there are scalar fields that can mimic that dependence.  
> **username_3**: @Allure For example: "For a wide range of parameters, this relativistic MOND theory is consistent with the CMB measurements from Planck. This happens because $c^2_\mathrm{ad}$ and $w$ are small enough so that $\Pi\to 0$ and we get dustlike evolution . . ." Basically they have a field that behaves as dust (i.e. dark matter) and presumably has the same abundance, although the PRL is extremely light on details and I cannot find the promised ("in preparation") ref. [104]. They do note that the abundance "is not (classically) predicted" so I assume the data will force it to the known DM abundance.  

---

### Answer by username_14 on 2024-03-20. Score: 15

We've extensively studied dark matter properties using the effects of its gravity. It's no different than the Sun, really. We've only know what the Sun is made of and where it gets its energy from for about a century, but we've known of its existence from the effects of that mass and energy for as long as there have been people.

> **Comments:**
> **username_15**: [Cecilia Payne-Gaposchkin](https://en.wikipedia.org/wiki/Cecilia_Payne-Gaposchkin) "proposed in her 1925 doctoral thesis that stars were composed primarily of hydrogen and helium. Her groundbreaking conclusion was initially rejected because it contradicted the scientific wisdom of the time, which held that there were no significant elemental differences between the Sun and Earth".  
> **username_12**: @pm: yes, but she was a woman in 1925. Einstein had to intercede for M. Curie, who's contributions far exceeded Cece's. And until 1960, female astronomers couldn't book time at Jodrell bank because the facility didn't have a ladies' room.  
> **username_15**: @Miss_Understands Indeed! As Wikipedia says, Henry Norris Russell initially convinced Cecilia to tone down her conclusions, and when he changed his mind 4 years later, mentioning her work, he was generally credited for the conclusions she reached. :(  
> **username_15**: Here are some other women who made important contributions to astronomy, mostly underrated. Sophia Brahe, Émilie du Châtelet, Caroline Herschel, Annie Jump Cannon, and Henrietta Swan Leavitt. In more modern times we have Margaret Burbidge, Jocelyn Bell Burnell, Michelle Chapront-Touzé, and Nicole Capitaine, but they've still had to work against gender bias. Also see *Pythagoras' Trousers* by Margaret Wertheim.  
> **username_12**: And the woman who explained mass loss after nuclear reactions, and the pulsar lady, and Hypathia of Alexandria.  

---

### Answer by username_10 on 2024-03-21. Score: 11

The presence of a third family of [massive quarks](https://en.wikipedia.org/wiki/Top_quark) was theoretically predicted in 1973, based on clues provided by other particle physics experiments.

The bottom quark was discovered in 1975, but the top quark was not revealed until 1995.

I think this is a reasonably apt analogy. There are plenty of observational clues that something called dark matter exists. These include: the dynamics of stars and gas in galaxies and the dynamics of galaxies in clusters; the gravitational lensing of light by clusters; the temperature fluctuations in the cosmic microwave background; the primordial abundance of helium and deuterium; and the way that cosmic structure has developed as the universe has evolved.

Attempts to work out what this dark matter is have proceeded in a scientific fashion - ruling out possibilities one-by-one whilst also making attempts at direct detection. No *proper scientist* ever makes the claim that dark matter *must* exist, and there are plenty of scientists out there suggesting and pursuing alternatives. Most, if not all, of these alternatives offer a less comprehensive explanation of observations than the unpalatable hypothesis that we do not yet understand what 80+% of the gravitational matter in the universe is.


---

### Answer by username_18 on 2024-03-22. Score: 4

Your question is a "wag the dog" one. Observations indicate that there is mass which is otherwise unaccounted for (and more specifically, 85% of the mass present in the universe). This is, then, by definition, what "dark matter" refers to.

Several of the other answers here already make this point, more or less, but I feel compelled to post an answer which points this out succinctly.

> **Comments:**
> **username_0**: I was under the impression that "dark matter" should be thought of as placeholder for a future theory that can explain a set of phenomena that we're currently confused about.   Stating explicitly that 85% of the universe is made up of dark matter seems to be jumping the gun.  
> **username_19**: @Zinn: No, placeholder is a fair term, but it’s not jumping the gun.  In fact it’s just opposite: we don’t need the place holder for 15% of the matter in the universe.  
> **username_3**: @Zinn To the extent that "dark matter" might be regarded as a placeholder term, it's a placeholder *for a more precise picture of what the matter is.* Not a placeholder for some alternative explanation that doesn't involve matter.  

---

### Answer by username_20 on 2024-03-21. Score: 0

Your question is actually spot on, and I disagree with the other answers here. There is indeed a contradiction. But the problem is not with the statement about the percentage of dark matter to matter, the problem is with the statement "We don't know what dark matter is".

We do. We do know what it is. It is that which makes up 85% of the matter in the universe.

The statement "We don't know what X *is*" is used FAR too much in science. It is a philosophical statement which can never be fully answered to exhaustion. We don't know what *electrons* are, not really. Are they fundamental point particles, emergent properties of a field, just a representation of an underlying mathematical symmetry, all of the above? We don't know what *viruses* are, are they nonliving, are they another branch of life, a different form of life entirely? We don't know even know what *HUMANS* are. Are neanderthals considered human, is a fetus a human, are genetically modified humans human? You can see how the question is essentially philosophical.

There is a lot we know about dark matter, there is a lot we don't. To say we don't know what it is simply because we are unsure of the exact subatomic makeup or relation to gravity, when we very well knew what things were for hundreds of years before we knew what gravity or an atom even was, is just silly.

There is indeed a contradiction.

> **Comments:**
> **username_3**: Totally agree with this. We know, to a large extent, how dark matter is described in terms of classical continuum mechanics. What we don't know is the *particle-level* description of dark matter. Dark matter is not even unique in that way: we don't know the particle-level description of gravity either, and does anyone object to talking about the effects of gravity?  
> **username_10**: We don't know whether it is (1) lots of massive particles, (2) far fewer primordial black holes, (3) not there at all. To me that is pretty much as clear a "we don't know what it is" as there could be.  
> **username_0**: @ProfRob it's the possibility of number (3) which makes the statement "dark matter makes up about 85% of the cosmos" seem presumptuous to me  
> **username_20**: @Zinn Possibility number (3) has pretty much been ruled out by both theory and observation, it really is a small, vocal minority which is still pushing various modifications of MOND as an explanation. It is one of those theories, like string theory, that can forever be modified with more and more parameters to continually make up for its inconsistency with what is observed.  
> **username_20**: @ScienceSnake That epistemological problem is exactly what my answer is attempting to avoid. Too often people claim something is unknown because it has not been exhaustively proven, but the thing is nothing can be. It is not presumptious to call something known with only partial knowledge, because nothing is known with *absolute* certainty. Sure, we know a lot more about electrons than dark matter, but we do actually know a heck of a lot about dark matter. We have gigabytes of astronomical data back it up, to put the bar at dark matter would exclude so much of the mundane we know far less of.  

---

### Answer by username_21 on 2024-03-21. Score: -1

Well, first off, given how contradictory recent studies have shown to be, MOND isn't exactly a theory that's backed up by much anymore. It is in kind of a foggy place at the moment. It could still be a possibility, as you have stated ; but there would be a lot of issues remaining before being able to consider it to its full extent.

Now as for just how one may be able to derive such a proportion despite their absolute ignorance regarding what this supposed (*supposed !*) dark matter exactly might be, or exactly how it should behave in the grand scheme of things ; you must remember that this is purely a theory, and at that, one that was derived from observation --as to fit it.

An offset between reality as we observe it to be and that which we compute through models based on relativity is present at most scales, and increases the greater the scale.
In order to "fix" these discrepancies, *some of* the leading theories on the matter (pun intended) derived the existence of some kind of "other" matter --that had mass, for certain, as that is how it would come to solve the issue in the first place, but which had to interact differently with "usual" (or regular) mass ; hence our inability to quite simply observe it.

Knowing this, it becomes obvious where this proportion arose from ; as this supposed "dark matter" is used as a kind of fix for models involving relativity to coincide with reality, a certain amount of it (which varies depending on the exact theory you consider to be the closest match) and only that amount would "solve" the issue.

Looking at the way elements of a galaxy spin around its center black hole for instance, it becomes apparent that either the model is missing something (hidden variables at a macroscopic scale), or that there is more to what we are currently able to observe.
The latter option is what dark matter variations aim to solve, through a type of matter that only loosely interacts with other kinds of matter.

---

Perhaps this could even be attributed to vacuum energy and relatively empty space's proximity to massive celestial bodies, stretching apart this energy of emptiness to different extents. (take this with a grain of salt, that last bit isn't backed up by anything as it is but a thought that has occurred to me which I haven't got to look into just yet)


---

