<p>The modern dark matter theory assumes that a spiral galaxy is a system of orbiting objects in the same way that a solar system is a system of orbiting objects.</p>

<p>That is not the case.  The stars in an elliptical galaxy are not <em>in orbit</em> around the galactic center.  They revolve around it, but they are not in an equilibrium state where they would be both falling into and coasting away from the central object at essentially the same speed.  All the stars are either falling in towards the center -- in a long spiral journey -- or are drifting out towards the intergalactic void in a long spiral journey. You could intuitively draw a circle on a photo of a spiral galaxy demarcating the border separating the ingoing from the outgoing stars.</p>

<p>It has to be remembered that our galaxy, for instance, is roughly a billion times larger in diameter than what might be determined to be the diameter of our solar system.  The motion of the stars, radially and otherwise, is <em>relatively</em> much, much slower.  Profoundly slower.</p>

<p>In our solar system, all the mass of the primordial rotating disk has long since either become part of the sun or has coasted outward.  The system is "cleared out" -- all except for a few planets whose very existence is close to being a miracle . A few lucky lumps and orbs, have, in the chaos of motion, somehow fallen into a highly improbable equilibrium state.  The sun has 98.5 per cent of all the mass in the solar system, and most of the rest is Jupiter, so the rest is really just debris.</p>

<p>In contrast, it takes a stupendous amount of time for a galaxy to clear out.  The stars would all have gone through their life cycle and would no longer be producing light.  We are watching matter inexorably falling in, and matter inexorably exiting (with maybe just a few stars ultimately ending up in an unlikely equilibrium state).</p>

<p>The stars in a galaxy initially got their roughly equivalent momentum from interactive gravitational forces.  But the point is, when we observe a spiral galaxy, we are always looking at something that is just at the beginning of a process that has already run its course in a solar system.  There is no need for 'dark matter' at the periphery of galaxy, because there is nothing being held in orbit.</p>

CC BY-SA 3.0
2016-10-02T08:24:48.587Z
Where do you get the idea that stars are not orbiting in the Galactic potential and are either falling in or travelling out of the Galaxy? Observations do not bear that out at all. The Sun and most of the stars around it are travelling mostly tangentially to the Galactic centre at about 220 km/s. Inward/ouward, up/down velocities are very small in comparison.
ProfRob
CC BY-SA 3.0
2016-10-02T12:33:51.180Z
I just added the clause "in a long spiral journey" in the second paragraph for clarity.  While most of a star's momentum appears tangential to the galactic center, any sustained inward or outward velocity means it's not in orbit.
T.D.
CC BY-SA 3.0
2016-10-02T12:44:41.747Z
Stars do not travel in spiral paths, since there is no force that compels them to do so. The orbits are approximately elliptical, with small epicyclic deviations. However, it is your comments about a demarcation between ingoing and outgoing stars (which doesn't exist) and that dark matter is not important at the periphery of a galaxy (the opposite is true) that earn you my downvote.
ProfRob
CC BY-SA 3.0
2016-10-02T13:17:38.967Z
The demarcation is not an actual feature of a galaxy but rather a description of the zone where ingoing and outgoing forces are close to parity. A galaxy should be regarded as being congruent with the rest of nature.  A spiral galaxy is not a whirlpool -- although it is in some ways similar, and it's not a wheel, although it is in some ways similar. The inner realm acts like something akin to a bathtub draining, and the outer realm is like a disintegrated, or, rather, a non-integrated wheel -- it's spinning, but it's flying apart.
T.D.
CC BY-SA 3.0
2016-10-02T15:31:10.713Z
What outgoing forces? There is only a gravitational force acting inwards. Stars in stable orbits are not in equilibrium - they have an acceleration; inwards. Your comments suggest you do not understand how orbits or circular motion work.
ProfRob
CC BY-SA 3.0
2016-10-02T17:42:50.957Z
What I do know is that if you want to put an object into orbit you have to exert a huge force on it -- a huge upward force, a huge outward force.   That force remains a component of the object's motion while it is in orbit.  Decrease it and the object eventually falls back in.  Increase it enough and the object gets further away forever.  An object accelerates as it falls because the direction of its momentum becomes more unequivocal, but an object in orbit remains at roughly the same speed because the direction of its momentum remains equivocal.
T.D.
CC BY-SA 3.0
2016-10-01T20:33:49.633Z
2016-10-02T12:18:48.017Z
2016-10-02T12:18:48.017Z
T.D.
<p>The question seems to be based on some misconceptions, so I'll try to clear them up.</p>

<h1>Dark matter is not the same as dark energy.</h1>

<p><a href="https://en.wikipedia.org/wiki/Dark_Matter" rel="noreferrer">Dark matter</a> and <a href="https://en.wikipedia.org/wiki/Dark_energy" rel="noreferrer">dark energy</a> are two completely separate and unrelated concepts. It's unfortunate that many articles trying to entice readers with tales of the unexplored frontiers of astronomy tend to use the phrase "dark matter and dark energy" quite a lot, typically with the buzzwords "mysterious," "unknown," and even "spooky."</p>

<p>Obviously, the best introduction here is to read the Wikipedia articles on dark matter and dark energy. You can also see the <a href="/questions/tagged/dark-matter" class="post-tag" title="show questions tagged 'dark-matter'" rel="tag">dark-matter</a> and <a href="/questions/tagged/dark-energy" class="post-tag" title="show questions tagged 'dark-energy'" rel="tag">dark-energy</a> tags (and the questions that use them) for more information. <a href="https://physics.stackexchange.com/">Physics</a> also has some good questions about these topics.</p>

<p>I'll give you the important points:</p>

<ul>
<li><p><strong>Dark matter . . .</strong></p>

<ul>
<li>. . . interacts via gravity</li>
<li>. . . has no electric charge (and so does not interact with light outside of possibly <a href="https://en.wikipedia.org/wiki/Gravitational_lens" rel="noreferrer">gravitational lensing</a>, which does not involve electric charge)</li>
<li>. . . may be composed of one (or more) of <a href="https://astronomy.stackexchange.com/questions/322/dark-matter-particle-candidates?rq=1">several hypothetical (and some not-so-hypothetical) particles</a>, such as axions or WIMPS (one of the more popular choices)</li>
<li>. . . is responsible for some stars <a href="https://en.wikipedia.org/wiki/Galaxy_rotation_curve" rel="noreferrer">moving faster than they should</a></li>
</ul></li>
<li><p><strong>Dark energy . . .</strong></p>

<ul>
<li>. . . is responsible for the acceleration of the expansion of the universe, though not the expansion of the universe</li>
<li>. . . can be represented by <a href="https://en.wikipedia.org/wiki/Cosmological_constant" rel="noreferrer">the cosmological constant</a></li>
</ul></li>
</ul>

<p>The one thing that the two share? We know very little about them.</p>

<h1>Dark energy does not solve as many problems in the Big Bang theory as you may think.</h1>

<p>In fact, it causes more problems than it solves.</p>

<p>You can come up with many different universes that begin with a Big Bang but don't have an accelerated expansion (not counting <a href="https://en.wikipedia.org/wiki/Inflation_(cosmology)" rel="noreferrer">inflation</a>, though you don't have to have that in a universe under some circumstances). They don't somehow suffer from any problems that universes with dark energy don't have. In fact, they're better because you don't have to somehow explain dark energy.</p>

<p>Courtesy of <a href="https://astronomy.stackexchange.com/questions/10117/does-dark-matter-really-exist/10120#comment13269_10120">Rob Jeffries</a>, here's one tidbit about the one fairly important role of dark energy in the Big Bang:</p>

<p>Our universe is close to flat (see the many papers by the WMAP team), and quite a lot of measurements have confirmed that we're not living on some wacky-shaped universe. The issue is that the measurements indicate that the universe isn't <em>quite</em> flat. Again, see the WMAP data. You can attribute part of the percentage offset from flatness to experimental error, or you can attribute it to the idea that perhaps the universe isn't so flat after all.</p>

<p>Dark energy provides an explanation for this inherent not-quite-flat-ness that characterizes our universe. In those other toy universes (without dark matter), you need to explain this near-flatness some other way. Not in our universe, it seems.</p>

<h1>Dark matter and dark energy aren't just theories.</h1>

<p>Well, sort of. Coming from someone like me (who is firmly on the "theorist" side of the scientific spectrum), that's quite a whopper. But from a certain point of view, it's completely accurate.</p>

<p>Dark matter is actually a theory. It's an explanation for some screwed-up galactic rotation curves, where some stars near the outer edges of a galaxy move much faster than they should.</p>

<p><a href="https://upload.wikimedia.org/wikipedia/commons/e/ec/M33_rotation_curve_HI.gif" rel="noreferrer"><img src="https://upload.wikimedia.org/wikipedia/commons/e/ec/M33_rotation_curve_HI.gif" alt=""></a></p>

<p>So it's actually a theory to explain a phenomenon.</p>

<p>Dark energy, on the other hand, is an experimental phenomenon. We don't have a good theory to explain it. But it's a phenomenon responsible for <a href="https://en.wikipedia.org/wiki/Dark_energy#Evidence_of_existence" rel="noreferrer">many different problems</a>. There's quite a lot of evidence that says, "Something odd is going on."</p>

<p>What I mean by all this is that there's a heck of a lot of evidence for them. Our theories of them were created to explain evidence, and not vice versa (evidence gathered to support or refute a theory).</p>

<hr>

<p>I advise, once again, that you look at the Wikipedia articles on both concepts to start learning more (especially the sections on evidence). Physics and Astronomy have plenty of good information. <a href="http://arxiv.org/" rel="noreferrer">arXiv</a> has many pre-prints that may be helpful. And there are lots of books, magazines, and (reputable) web sites that can give you even more detailed information.</p>

CC BY-SA 3.0
2015-03-14T08:08:45.027Z
Dark matter has far more going for it than galaxy rotation curves: cluster velocity dispersions; the growth of cosmological structure; primordial abundances from nucleosynthesis; gravitational lensing in clusters; holding dwarf spheroidal galaxies together.
ProfRob
CC BY-SA 3.0
2015-03-14T08:16:54.383Z
Possibly semantics, but I do think that dark energy solves a problem with the big bang model. Without it one requires an explanation for why the universe is nearly flat, but not flat - which is inherently unlikely. Very-close-to flatness is predicted by inflationary models and cannot be achieved with matter plus dark matter alone. +1 for having the patience to provide an answer.
ProfRob
CC BY-SA 3.0
2015-03-14T22:03:43.227Z
@RobJeffries A) Thanks for weighing in; I had hoped you'd see the question. B) I was tempted to go into the other details (especially the lensing, as that can make for some cool effects that get the point across), but I wanted to stick to the basics, as I have a tendency to be a bit long-winded. C) I think you're absolutely right to bring up the flatness issue; I didn't know about it.
HDE 226868
CC BY-SA 3.0
2016-03-20T16:44:11.483Z
You have misunderstood my comment. The Planck and WMAP data show that the universe *is* flat (within their error bars). Without dark energy, $\Omega \sim 0.32$, which is what I mean by nearly flat, but not flat. Inflation is the theory for why the universe is almost exactly flat. But if you have inflation then the universe must be flat and so you need to add some form of energy density to the dark and normal matter to make $\Omega=1$. That role is played by dark energy. Without it, you have two problems (cont.)
ProfRob
CC BY-SA 3.0
2016-03-20T16:55:04.983Z
(i) Why does the CMB indicate that $\Omega=1$ when dark + normal matter suggests $\Omega \sim 0.32$.  (ii) Even if you disregard the CMB evidence, it is very hard to arrange to have a universe with $\Omega \sim 1$ but not exactly 1. The so-called "flatness problem".
ProfRob
CC BY-SA 3.0
2015-03-13T22:22:43.167Z
2015-03-14T22:09:15.560Z
2015-03-14T22:09:15.560Z
HDE 226868
<p>Does dark matter really exist or is just a made-up theory in order to cover up the loopholes in the Big Bang theory, and also to explain why universe is expanding at an accelerated rate?</p>

CC BY-SA 3.0
2015-03-13T19:12:46.973Z
To start with, you've confused dark matter (which would bring the universe together) with dark energy (which is theorized to be the reason for continued expansion).
Donald.McLean
CC BY-SA 3.0
2015-03-13T21:23:02.763Z
when you say 'made up theory', do you realise that every theory must have been made up at some point? also there is definitely evidence supporting its existance.
stanley dodds
<dark-matter>
Does "Dark Matter" really exist?
sushil kumar
# Does "Dark Matter" really exist?

(Asked by: username_0 on 2015-03-13)

Does dark matter really exist or is just a made-up theory in order to cover up the loopholes in the Big Bang theory, and also to explain why universe is expanding at an accelerated rate?

**Tags:** dark-matter
> **Comments:**
> **username_1**: To start with, you've confused dark matter (which would bring the universe together) with dark energy (which is theorized to be the reason for continued expansion).  
> **username_2**: when you say 'made up theory', do you realise that every theory must have been made up at some point? also there is definitely evidence supporting its existance.  

---

## 2 answers

### Answer by username_5 on 2015-03-13. Score: 8 | Accepted answer

The question seems to be based on some misconceptions, so I'll try to clear them up.

# Dark matter is not the same as dark energy.

[Dark matter](https://en.wikipedia.org/wiki/Dark_Matter) and [dark energy](https://en.wikipedia.org/wiki/Dark_energy) are two completely separate and unrelated concepts. It's unfortunate that many articles trying to entice readers with tales of the unexplored frontiers of astronomy tend to use the phrase "dark matter and dark energy" quite a lot, typically with the buzzwords "mysterious," "unknown," and even "spooky."

Obviously, the best introduction here is to read the Wikipedia articles on dark matter and dark energy. You can also see the [dark-matter](/questions/tagged/dark-matter "show questions tagged 'dark-matter'") and [dark-energy](/questions/tagged/dark-energy "show questions tagged 'dark-energy'") tags (and the questions that use them) for more information. [Physics](https://physics.stackexchange.com/) also has some good questions about these topics.

I'll give you the important points:

* **Dark matter . . .**

  + . . . interacts via gravity
  + . . . has no electric charge (and so does not interact with light outside of possibly [gravitational lensing](https://en.wikipedia.org/wiki/Gravitational_lens), which does not involve electric charge)
  + . . . may be composed of one (or more) of [several hypothetical (and some not-so-hypothetical) particles](https://astronomy.stackexchange.com/questions/322/dark-matter-particle-candidates?rq=1), such as axions or WIMPS (one of the more popular choices)
  + . . . is responsible for some stars [moving faster than they should](https://en.wikipedia.org/wiki/Galaxy_rotation_curve)
* **Dark energy . . .**

  + . . . is responsible for the acceleration of the expansion of the universe, though not the expansion of the universe
  + . . . can be represented by [the cosmological constant](https://en.wikipedia.org/wiki/Cosmological_constant)

The one thing that the two share? We know very little about them.

# Dark energy does not solve as many problems in the Big Bang theory as you may think.

In fact, it causes more problems than it solves.

You can come up with many different universes that begin with a Big Bang but don't have an accelerated expansion (not counting [inflation](https://en.wikipedia.org/wiki/Inflation_(cosmology)), though you don't have to have that in a universe under some circumstances). They don't somehow suffer from any problems that universes with dark energy don't have. In fact, they're better because you don't have to somehow explain dark energy.

Courtesy of [Rob Jeffries](https://astronomy.stackexchange.com/questions/10117/does-dark-matter-really-exist/10120#comment13269_10120), here's one tidbit about the one fairly important role of dark energy in the Big Bang:

Our universe is close to flat (see the many papers by the WMAP team), and quite a lot of measurements have confirmed that we're not living on some wacky-shaped universe. The issue is that the measurements indicate that the universe isn't *quite* flat. Again, see the WMAP data. You can attribute part of the percentage offset from flatness to experimental error, or you can attribute it to the idea that perhaps the universe isn't so flat after all.

Dark energy provides an explanation for this inherent not-quite-flat-ness that characterizes our universe. In those other toy universes (without dark matter), you need to explain this near-flatness some other way. Not in our universe, it seems.

# Dark matter and dark energy aren't just theories.

Well, sort of. Coming from someone like me (who is firmly on the "theorist" side of the scientific spectrum), that's quite a whopper. But from a certain point of view, it's completely accurate.

Dark matter is actually a theory. It's an explanation for some screwed-up galactic rotation curves, where some stars near the outer edges of a galaxy move much faster than they should.

[![](https://upload.wikimedia.org/wikipedia/commons/e/ec/M33_rotation_curve_HI.gif)](https://upload.wikimedia.org/wikipedia/commons/e/ec/M33_rotation_curve_HI.gif)

So it's actually a theory to explain a phenomenon.

Dark energy, on the other hand, is an experimental phenomenon. We don't have a good theory to explain it. But it's a phenomenon responsible for [many different problems](https://en.wikipedia.org/wiki/Dark_energy#Evidence_of_existence). There's quite a lot of evidence that says, "Something odd is going on."

What I mean by all this is that there's a heck of a lot of evidence for them. Our theories of them were created to explain evidence, and not vice versa (evidence gathered to support or refute a theory).

---

I advise, once again, that you look at the Wikipedia articles on both concepts to start learning more (especially the sections on evidence). Physics and Astronomy have plenty of good information. [arXiv](http://arxiv.org/) has many pre-prints that may be helpful. And there are lots of books, magazines, and (reputable) web sites that can give you even more detailed information.

> **Comments:**
> **username_4**: Dark matter has far more going for it than galaxy rotation curves: cluster velocity dispersions; the growth of cosmological structure; primordial abundances from nucleosynthesis; gravitational lensing in clusters; holding dwarf spheroidal galaxies together.  
> **username_4**: Possibly semantics, but I do think that dark energy solves a problem with the big bang model. Without it one requires an explanation for why the universe is nearly flat, but not flat - which is inherently unlikely. Very-close-to flatness is predicted by inflationary models and cannot be achieved with matter plus dark matter alone. +1 for having the patience to provide an answer.  
> **username_5**: @RobJeffries A) Thanks for weighing in; I had hoped you'd see the question. B) I was tempted to go into the other details (especially the lensing, as that can make for some cool effects that get the point across), but I wanted to stick to the basics, as I have a tendency to be a bit long-winded. C) I think you're absolutely right to bring up the flatness issue; I didn't know about it.  
> **username_4**: You have misunderstood my comment. The Planck and WMAP data show that the universe *is* flat (within their error bars). Without dark energy, $\Omega \sim 0.32$, which is what I mean by nearly flat, but not flat. Inflation is the theory for why the universe is almost exactly flat. But if you have inflation then the universe must be flat and so you need to add some form of energy density to the dark and normal matter to make $\Omega=1$. That role is played by dark energy. Without it, you have two problems (cont.)  
> **username_4**: (i) Why does the CMB indicate that $\Omega=1$ when dark + normal matter suggests $\Omega \sim 0.32$.  (ii) Even if you disregard the CMB evidence, it is very hard to arrange to have a universe with $\Omega \sim 1$ but not exactly 1. The so-called "flatness problem".  

---

### Answer by username_3 on 2016-10-01. Score: -3

The modern dark matter theory assumes that a spiral galaxy is a system of orbiting objects in the same way that a solar system is a system of orbiting objects.

That is not the case. The stars in an elliptical galaxy are not *in orbit* around the galactic center. They revolve around it, but they are not in an equilibrium state where they would be both falling into and coasting away from the central object at essentially the same speed. All the stars are either falling in towards the center -- in a long spiral journey -- or are drifting out towards the intergalactic void in a long spiral journey. You could intuitively draw a circle on a photo of a spiral galaxy demarcating the border separating the ingoing from the outgoing stars.

It has to be remembered that our galaxy, for instance, is roughly a billion times larger in diameter than what might be determined to be the diameter of our solar system. The motion of the stars, radially and otherwise, is *relatively* much, much slower. Profoundly slower.

In our solar system, all the mass of the primordial rotating disk has long since either become part of the sun or has coasted outward. The system is "cleared out" -- all except for a few planets whose very existence is close to being a miracle . A few lucky lumps and orbs, have, in the chaos of motion, somehow fallen into a highly improbable equilibrium state. The sun has 98.5 per cent of all the mass in the solar system, and most of the rest is Jupiter, so the rest is really just debris.

In contrast, it takes a stupendous amount of time for a galaxy to clear out. The stars would all have gone through their life cycle and would no longer be producing light. We are watching matter inexorably falling in, and matter inexorably exiting (with maybe just a few stars ultimately ending up in an unlikely equilibrium state).

The stars in a galaxy initially got their roughly equivalent momentum from interactive gravitational forces. But the point is, when we observe a spiral galaxy, we are always looking at something that is just at the beginning of a process that has already run its course in a solar system. There is no need for 'dark matter' at the periphery of galaxy, because there is nothing being held in orbit.

> **Comments:**
> **username_4**: Where do you get the idea that stars are not orbiting in the Galactic potential and are either falling in or travelling out of the Galaxy? Observations do not bear that out at all. The Sun and most of the stars around it are travelling mostly tangentially to the Galactic centre at about 220 km/s. Inward/ouward, up/down velocities are very small in comparison.  
> **username_3**: I just added the clause "in a long spiral journey" in the second paragraph for clarity.  While most of a star's momentum appears tangential to the galactic center, any sustained inward or outward velocity means it's not in orbit.  
> **username_4**: Stars do not travel in spiral paths, since there is no force that compels them to do so. The orbits are approximately elliptical, with small epicyclic deviations. However, it is your comments about a demarcation between ingoing and outgoing stars (which doesn't exist) and that dark matter is not important at the periphery of a galaxy (the opposite is true) that earn you my downvote.  
> **username_3**: The demarcation is not an actual feature of a galaxy but rather a description of the zone where ingoing and outgoing forces are close to parity. A galaxy should be regarded as being congruent with the rest of nature.  A spiral galaxy is not a whirlpool -- although it is in some ways similar, and it's not a wheel, although it is in some ways similar. The inner realm acts like something akin to a bathtub draining, and the outer realm is like a disintegrated, or, rather, a non-integrated wheel -- it's spinning, but it's flying apart.  
> **username_4**: What outgoing forces? There is only a gravitational force acting inwards. Stars in stable orbits are not in equilibrium - they have an acceleration; inwards. Your comments suggest you do not understand how orbits or circular motion work.  
> **username_3**: What I do know is that if you want to put an object into orbit you have to exert a huge force on it -- a huge upward force, a huge outward force.   That force remains a component of the object's motion while it is in orbit.  Decrease it and the object eventually falls back in.  Increase it enough and the object gets further away forever.  An object accelerates as it falls because the direction of its momentum becomes more unequivocal, but an object in orbit remains at roughly the same speed because the direction of its momentum remains equivocal.  

---

