<p>Multiple theories and hypotheses have been proposed as an alternative to dark matter (DM). The most popular are, arguably,</p>

<ul>
<li><a href="https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics" rel="noreferrer">MOND</a> (MOdified Newtonian Dynamics):<br>A term for various theories where the gravitational force falls off less steeply than $1/r^2$ at large distances.</li>
<li><a href="https://en.wikipedia.org/wiki/Tensor%E2%80%93vector%E2%80%93scalar_gravity" rel="noreferrer">TeVeS</a> (Tensor–vector–scalar):<br>A relativistic generalization of MOND.</li>
<li><a href="https://en.wikipedia.org/wiki/F(R)_gravity" rel="noreferrer">f(R) gravity</a>:<br>A generalization of general relativity, where the so-called Ricci scalar — which describes the curvature of space(time) — may take different forms than just a number.</li>
<li><a href="https://en.wikipedia.org/wiki/Entropic_gravity" rel="noreferrer">Entropic gravity</a>:<br>A string-theory-based theory describing gravity not as a fundamental interaction, but as a so-called entropic force.</li>
</ul>

<p>Note, however, that DM is <em>not</em> just "a fudge factor to explain weird dynamics". What makes most (but not all) physicists believe that DM exists is, I think, that several <em>independent</em> observations yield consistent results, not only qualitatively, but quantitatively. In addition to <a href="https://en.wikipedia.org/wiki/Galaxy_rotation_curve" rel="noreferrer">rotation curves</a> of spiral galaxies, or <a href="https://en.wikipedia.org/wiki/Velocity_dispersion" rel="noreferrer">velocity dispersions</a> in stellar clusters, elliptical galaxies, and galaxy clusters, you have for instance</p>

<ul>
<li><p><a href="https://en.wikipedia.org/wiki/Gravitational_lens" rel="noreferrer">Gravitational lensing</a>:<br>Massive clusters of galaxies "bend" space around them, causing the light from background sources to diverge from a straight path. Measuring this divergence tells us the mass of the foreground object, which can then be compared to the visible mass..</p></li>
<li><p><a href="https://en.wikipedia.org/wiki/Cosmic_microwave_background" rel="noreferrer">Cosmic microwave background</a>:<br>The patterns in the inhomogeneities of the background radiation is sensitive to the amounts of both dark and "normal" matter.</p></li>
<li><p><a href="https://en.wikipedia.org/wiki/Structure_formation" rel="noreferrer">Structure formation</a>:<br>DM explains how matter may collapse and form galaxies a few 100 million years after Big Bang, before the Universe expanded too much.</p></li>
<li><p><a href="https://en.wikipedia.org/wiki/Type_Ia_supernova" rel="noreferrer">Supernova observations</a>:<br>The way that the brightness of distant supernovae decreases with redshift depends on the expansion history of the Universe, which in turn depends on the densities of the various components of the Universe. A certain amount of matter is needed to explain the expansion, and this amount is larger than the observed amount by the same factor that the other experiments yield.</p></li>
</ul>

<p>Some of the above phenomena are explained by the alternative theories as well, but in general several ad hoc hypotheses have had to be made. In my view (but I am no expert on this!), DM is simply the simplest explanation, and so, by Occam's Razor, is favored by most physicists. I think the theory, unfairly, has a reputation for being "some magic that astronomers pulled out of their hat to explain what they don't understand, and which we made sure can't be observed so you can't prove us wrong". But firstly, postulating an unknown object to explain some phenomenon is quite normal in physics — this is how e.g. Neptune and several elementary particles were found. And secondly, there are several candidates for what DM might be, and at least some of these make falsifiable predictions. If DM is a particle, then that particle might produce a signal that can be observed. There are many efforts to search for a DM annihilation signal, which is expected to be in the (tens of) GeV region (e.g. <a href="https://arxiv.org/abs/1103.3266" rel="noreferrer">H.E.S.S Collaboration 2011</a> and <a href="https://arxiv.org/abs/1610.03840" rel="noreferrer">Cui et al. 2017</a>). There are also efforts for direct detections — see e.g. <a href="https://arxiv.org/abs/1709.00688" rel="noreferrer">Liu et al. (2017)</a> for a review.</p>

<p>One of the most compelling pieces of evidence yet for DM came, I think, with the observation of the <a href="https://en.wikipedia.org/wiki/Bullet_Cluster" rel="noreferrer">Bullet Cluster</a> (and subsequently many other clusters), where lensing clearly shows how unaccounted-for mass follows the collisionless galaxies, but not the collisional gas, as seen in this beautiful image:</p>

<p><a href="https://i.sstatic.net/BTTDK.jpg" rel="noreferrer"><img src="https://i.sstatic.net/BTTDK.jpg" alt="Bullet"></a>
<sup>The Bullet Cluster. <em>Pink</em> shows X-rays from the gas, whereas <em>blue</em> shows where the mass is concentrated. Credit: <a href="https://svs.gsfc.nasa.gov/30094" rel="noreferrer">NASA/CHANDRA</a>.</sup></p>

CC BY-SA 4.0
2018-08-21T10:32:07.373Z
Thanks for your answer! Two questions. (1) Do the theories in your first set of bullet points fail to explain the phenomena in your second?  For example, doesn't the amount of gravitational lensing correlate with the amount of force needed to explain galaxy rotation, so that both dark matter and something like f(R) that modifies the dynamics, accommodate the data?  (2) How do the supernova observations fit in, since DM is hypothesized to not interact with light?
mbsq
CC BY-SA 4.0
2018-08-21T12:54:38.070Z
@mbsq I must say that I'm no expert in alternative theories of DM, but in general, alternative theories have had to come up with many ad hoc theories to explain the phenomena mentioned above, as well as other phenomena. In my view, the deathblow came with the observation of the [Bullet Cluster](https://en.wikipedia.org/wiki/Bullet_Cluster) (and subsequently many other clusters), where lensing clearly shows how unaccounted-for mass follows the collisionless galaxies, but not the collisional gas.
pela
CC BY-SA 4.0
2018-08-21T12:58:39.917Z
Wrt. your second question, the way that SN brightness decreases with redshift depends on the expansion history of the Universe, which in turn depends on the densities of the various components of the Universe. A certain amount of matter is needed to explain the expansion, and this amount is larger than the observed amount by the same factor that the other experiments yield.
pela
CC BY-SA 4.0
2018-08-21T12:59:41.343Z
Interesting. My question is motivated by Occam's Razor-- If stuff behaves like this, maybe we should just describe how stuff behaves, rather than postulating some unobservable other stuff that makes it so.  But if there are observable ways to test the invisible-stuff theory against the different-dynamics theory like this Bullet Cluster phenomenon, then great!
mbsq
CC BY-SA 4.0
2018-08-21T13:06:19.490Z
@mbsq I think that exactly Occam's razor is the best reason for believing in DM rather than an alternative: It is the simplest explanation. And I don't think it's a particularly outrageous idea — just a particle that is so small that the probability of interacting with other particles is vanishingly small. But it is by no means the first time that science has postulated some unobservable thing to account for some weird phenomena, which later proved to be correct, e.g. the planet Neptune, as well as a multitude of elementary particles.
pela
CC BY-SA 4.0
2018-08-21T13:35:11.623Z
Yes I suppose it's simple, but it has a kind of "fudge factor" appearance.  Especially when we can't go looking for it, like Neptune.
mbsq
CC BY-SA 4.0
2018-08-21T14:21:22.717Z
@mbsq Actually, that's not really true, I think. There are several candidates for what DM might be, and at least some of these make falsifiable predictions. If it's really a particle, then that particle might produce a signal that can be observed. There are many efforts to search for a DM annihilation signal, which is expected to be in the GeV region. There are also efforts for direct detections — see e.g. [Liu+ 2017](https://arxiv.org/abs/1709.00688) for a review.
pela
CC BY-SA 4.0
2018-08-21T16:02:57.307Z
Since we don't know what DM is, is the phrase "dark matter" a placeholder for *something* that satisfies **all** the constraints you listed?  (It's just that MOND, TeVeS, etc just don't satisfy all the constraints.)
RonJohn
CC BY-SA 4.0
2018-08-21T18:49:13.780Z
@RonJohn Yes, I suppose you could say that. Any explanation that involves some sort of particle, be it baryonic or non-baryonic, as opposed to modifying existing theories of gravity/general relativity, would be called a dark matter theory.
pela
CC BY-SA 4.0
2023-05-26T21:50:14.690Z
@DaveTheWave Although we don't know what dark matter is, we do know that it is definitely not antimatter. If it were, it would annihilate with matter, producing extreme amounts of X-rays.
pela
CC BY-SA 4.0
2023-05-27T09:22:34.610Z
@DaveTheWave Well yes, you can conjecture any teapot you'd like, but that's not science. An extra property or charge would be incompatible with virtually all observations and experimental results within particle physics. Antimatter is not dark: It emits light just like matter, and if matter and antimatter meet, it annihilates and emits X-rays. If you were to place a galaxy of matter, in a halo of antimatter, it would be the most spectacular firework ever :)
pela
CC BY-SA 4.0
2018-08-21T10:25:32.250Z
2018-08-21T19:53:18.823Z
2018-08-21T19:53:18.823Z
pela
<p>As I understand, the evidence for dark matter is the observation of the gravitational dynamics of objects around the size of galaxies and above.  They move in a way such that the equations imply there is more mass than what we can see.  There has never been any direct observation of dark matter, and our theories tell us it cannot be composed of any elementary particles from the standard model.  Furthermore, there is no observation which suggests that dark matter is unevenly distributed, that some galaxies consisting of roughly the same amount of normal matter have varying amounts of dark matter.</p>

<p>I wonder if therefore an alternative explanation for the phenomena is that we have the wrong dynamical equations for gravity.  Just as Newtonian gravitation is a good approximation to general relativity at low masses/energies, perhaps general relativity is a good approximation to the true gravitational theory at sub-galactic distances.  Maybe when distances become very large we start seeing that gravatational force is described by slightly different curves than what general relativity predicts.</p>

<p>Is there any good evidence speaking against this alternative approach?</p>

<p>The reason this question is different from the "possible duplicate" is that the other question asks if the source of the effect attributed to dark matter is parallel universes, rather than different dynamical laws.  However, the answer there does mention these alternative gravitational theories.</p>

<p>I realize now that this is not a new idea, and I thank the commenters and answerers for pointing me to existing research.  However, this part of StackExchange does not bill itself as an "experts only" forum, so I don't think the downvotes are fair.  In mathematics for example, there is one forum for experts, and another for everyone.  I'm an expert in something, but not astronomy.  Thanks.</p>

CC BY-SA 4.0
2018-08-21T09:05:14.717Z
Yes, there are plenty. Googling something like "alternative explanations for dark matter" will lead to numerous descriptions of e.g. MOND, TeVeS, f(R) gravity, and entropic gravity. Note, however, that DM is _not_ just "something that explains weird dynamics". Gravitational lensing and observations of the cosmic microwave background yield results consistent with those obtained from studying the dynamics of galaxies and clusters.
pela
CC BY-SA 4.0
2018-08-21T09:12:15.223Z
Thank you.  f(R) gravity looks like the kind of thing I was imagining.
mbsq
CC BY-SA 4.0
2018-08-21T10:12:29.017Z
Hard to know what you are asking about. Many scientists are investigating alternatives to dark matter. Try googling "alternatives to dark matter".
ProfRob
CC BY-SA 4.0
2018-08-21T10:13:34.990Z
There is also the issue that the universe is geometrically flat but apparently contains only 5% of the density required to do so in the form of "normal" matter.
ProfRob
CC BY-SA 4.0
2018-08-21T10:14:43.453Z
How is it hard to know what I'm asking? It's not very helpful to say "google it."
mbsq
CC BY-SA 4.0
2018-08-21T12:49:32.130Z
The Wikipedia article on MOND indicates some of the issues with MOND theory:  https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics#Outstanding_problems_for_MOND
userLTK
CC BY-SA 4.0
2018-08-21T13:06:44.773Z
[Relevant xkcd](https://xkcd.com/1758/)
Sir Cumference
CC BY-SA 4.0
2018-08-21T15:42:01.383Z
Your "maybe" statments should be deleted.  The whole reason for calling these things  "dark" is just to indicate it's an unknown thing of unknown material (or energy, or field).  Of course alternative things might turn out to solve the mismatches!
Carl Witthoft
CC BY-SA 4.0
2018-08-21T15:44:17.957Z
I don't see why. It's an alternative theory. More unseen mass vs. different dynamical laws.
mbsq
<gravity><dark-matter>
Could dark matter be wrong?
mbsq
# Could dark matter be wrong?

(Asked by: username_0 on 2018-08-21)

As I understand, the evidence for dark matter is the observation of the gravitational dynamics of objects around the size of galaxies and above. They move in a way such that the equations imply there is more mass than what we can see. There has never been any direct observation of dark matter, and our theories tell us it cannot be composed of any elementary particles from the standard model. Furthermore, there is no observation which suggests that dark matter is unevenly distributed, that some galaxies consisting of roughly the same amount of normal matter have varying amounts of dark matter.

I wonder if therefore an alternative explanation for the phenomena is that we have the wrong dynamical equations for gravity. Just as Newtonian gravitation is a good approximation to general relativity at low masses/energies, perhaps general relativity is a good approximation to the true gravitational theory at sub-galactic distances. Maybe when distances become very large we start seeing that gravatational force is described by slightly different curves than what general relativity predicts.

Is there any good evidence speaking against this alternative approach?

The reason this question is different from the "possible duplicate" is that the other question asks if the source of the effect attributed to dark matter is parallel universes, rather than different dynamical laws. However, the answer there does mention these alternative gravitational theories.

I realize now that this is not a new idea, and I thank the commenters and answerers for pointing me to existing research. However, this part of StackExchange does not bill itself as an "experts only" forum, so I don't think the downvotes are fair. In mathematics for example, there is one forum for experts, and another for everyone. I'm an expert in something, but not astronomy. Thanks.

**Tags:** gravity, dark-matter
> **Comments:**
> **username_1**: Yes, there are plenty. Googling something like "alternative explanations for dark matter" will lead to numerous descriptions of e.g. MOND, TeVeS, f(R) gravity, and entropic gravity. Note, however, that DM is _not_ just "something that explains weird dynamics". Gravitational lensing and observations of the cosmic microwave background yield results consistent with those obtained from studying the dynamics of galaxies and clusters.  
> **username_0**: Thank you.  f(R) gravity looks like the kind of thing I was imagining.  
> **username_2**: Hard to know what you are asking about. Many scientists are investigating alternatives to dark matter. Try googling "alternatives to dark matter".  
> **username_2**: There is also the issue that the universe is geometrically flat but apparently contains only 5% of the density required to do so in the form of "normal" matter.  
> **username_0**: How is it hard to know what I'm asking? It's not very helpful to say "google it."  
> **username_3**: The Wikipedia article on MOND indicates some of the issues with MOND theory:  https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics#Outstanding_problems_for_MOND  
> **username_4**: [Relevant xkcd](https://xkcd.com/1758/)  
> **username_5**: Your "maybe" statments should be deleted.  The whole reason for calling these things  "dark" is just to indicate it's an unknown thing of unknown material (or energy, or field).  Of course alternative things might turn out to solve the mismatches!  
> **username_0**: I don't see why. It's an alternative theory. More unseen mass vs. different dynamical laws.  

---

## 1 answers

### Answer by username_1 on 2018-08-21. Score: 25 | Accepted answer

Multiple theories and hypotheses have been proposed as an alternative to dark matter (DM). The most popular are, arguably,

* [MOND](https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics) (MOdified Newtonian Dynamics):  
  A term for various theories where the gravitational force falls off less steeply than $1/r^2$ at large distances.
* [TeVeS](https://en.wikipedia.org/wiki/Tensor%E2%80%93vector%E2%80%93scalar_gravity) (Tensor–vector–scalar):  
  A relativistic generalization of MOND.
* [f(R) gravity](https://en.wikipedia.org/wiki/F(R)_gravity):  
  A generalization of general relativity, where the so-called Ricci scalar — which describes the curvature of space(time) — may take different forms than just a number.
* [Entropic gravity](https://en.wikipedia.org/wiki/Entropic_gravity):  
  A string-theory-based theory describing gravity not as a fundamental interaction, but as a so-called entropic force.

Note, however, that DM is *not* just "a fudge factor to explain weird dynamics". What makes most (but not all) physicists believe that DM exists is, I think, that several *independent* observations yield consistent results, not only qualitatively, but quantitatively. In addition to [rotation curves](https://en.wikipedia.org/wiki/Galaxy_rotation_curve) of spiral galaxies, or [velocity dispersions](https://en.wikipedia.org/wiki/Velocity_dispersion) in stellar clusters, elliptical galaxies, and galaxy clusters, you have for instance

* [Gravitational lensing](https://en.wikipedia.org/wiki/Gravitational_lens):  
  Massive clusters of galaxies "bend" space around them, causing the light from background sources to diverge from a straight path. Measuring this divergence tells us the mass of the foreground object, which can then be compared to the visible mass..
* [Cosmic microwave background](https://en.wikipedia.org/wiki/Cosmic_microwave_background):  
  The patterns in the inhomogeneities of the background radiation is sensitive to the amounts of both dark and "normal" matter.
* [Structure formation](https://en.wikipedia.org/wiki/Structure_formation):  
  DM explains how matter may collapse and form galaxies a few 100 million years after Big Bang, before the Universe expanded too much.
* [Supernova observations](https://en.wikipedia.org/wiki/Type_Ia_supernova):  
  The way that the brightness of distant supernovae decreases with redshift depends on the expansion history of the Universe, which in turn depends on the densities of the various components of the Universe. A certain amount of matter is needed to explain the expansion, and this amount is larger than the observed amount by the same factor that the other experiments yield.

Some of the above phenomena are explained by the alternative theories as well, but in general several ad hoc hypotheses have had to be made. In my view (but I am no expert on this!), DM is simply the simplest explanation, and so, by Occam's Razor, is favored by most physicists. I think the theory, unfairly, has a reputation for being "some magic that astronomers pulled out of their hat to explain what they don't understand, and which we made sure can't be observed so you can't prove us wrong". But firstly, postulating an unknown object to explain some phenomenon is quite normal in physics — this is how e.g. Neptune and several elementary particles were found. And secondly, there are several candidates for what DM might be, and at least some of these make falsifiable predictions. If DM is a particle, then that particle might produce a signal that can be observed. There are many efforts to search for a DM annihilation signal, which is expected to be in the (tens of) GeV region (e.g. [H.E.S.S Collaboration 2011](https://arxiv.org/abs/1103.3266) and [Cui et al. 2017](https://arxiv.org/abs/1610.03840)). There are also efforts for direct detections — see e.g. [Liu et al. (2017)](https://arxiv.org/abs/1709.00688) for a review.

One of the most compelling pieces of evidence yet for DM came, I think, with the observation of the [Bullet Cluster](https://en.wikipedia.org/wiki/Bullet_Cluster) (and subsequently many other clusters), where lensing clearly shows how unaccounted-for mass follows the collisionless galaxies, but not the collisional gas, as seen in this beautiful image:

[![Bullet](https://i.sstatic.net/BTTDK.jpg)](https://i.sstatic.net/BTTDK.jpg)
The Bullet Cluster. *Pink* shows X-rays from the gas, whereas *blue* shows where the mass is concentrated. Credit: [NASA/CHANDRA](https://svs.gsfc.nasa.gov/30094).

> **Comments:**
> **username_0**: Thanks for your answer! Two questions. (1) Do the theories in your first set of bullet points fail to explain the phenomena in your second?  For example, doesn't the amount of gravitational lensing correlate with the amount of force needed to explain galaxy rotation, so that both dark matter and something like f(R) that modifies the dynamics, accommodate the data?  (2) How do the supernova observations fit in, since DM is hypothesized to not interact with light?  
> **username_1**: @mbsq I must say that I'm no expert in alternative theories of DM, but in general, alternative theories have had to come up with many ad hoc theories to explain the phenomena mentioned above, as well as other phenomena. In my view, the deathblow came with the observation of the [Bullet Cluster](https://en.wikipedia.org/wiki/Bullet_Cluster) (and subsequently many other clusters), where lensing clearly shows how unaccounted-for mass follows the collisionless galaxies, but not the collisional gas.  
> **username_1**: Wrt. your second question, the way that SN brightness decreases with redshift depends on the expansion history of the Universe, which in turn depends on the densities of the various components of the Universe. A certain amount of matter is needed to explain the expansion, and this amount is larger than the observed amount by the same factor that the other experiments yield.  
> **username_0**: Interesting. My question is motivated by Occam's Razor-- If stuff behaves like this, maybe we should just describe how stuff behaves, rather than postulating some unobservable other stuff that makes it so.  But if there are observable ways to test the invisible-stuff theory against the different-dynamics theory like this Bullet Cluster phenomenon, then great!  
> **username_1**: @mbsq I think that exactly Occam's razor is the best reason for believing in DM rather than an alternative: It is the simplest explanation. And I don't think it's a particularly outrageous idea — just a particle that is so small that the probability of interacting with other particles is vanishingly small. But it is by no means the first time that science has postulated some unobservable thing to account for some weird phenomena, which later proved to be correct, e.g. the planet Neptune, as well as a multitude of elementary particles.  
> **username_0**: Yes I suppose it's simple, but it has a kind of "fudge factor" appearance.  Especially when we can't go looking for it, like Neptune.  
> **username_1**: @mbsq Actually, that's not really true, I think. There are several candidates for what DM might be, and at least some of these make falsifiable predictions. If it's really a particle, then that particle might produce a signal that can be observed. There are many efforts to search for a DM annihilation signal, which is expected to be in the GeV region. There are also efforts for direct detections — see e.g. [Liu+ 2017](https://arxiv.org/abs/1709.00688) for a review.  
> **username_6**: Since we don't know what DM is, is the phrase "dark matter" a placeholder for *something* that satisfies **all** the constraints you listed?  (It's just that MOND, TeVeS, etc just don't satisfy all the constraints.)  
> **username_1**: @RonJohn Yes, I suppose you could say that. Any explanation that involves some sort of particle, be it baryonic or non-baryonic, as opposed to modifying existing theories of gravity/general relativity, would be called a dark matter theory.  
> **username_1**: @DaveTheWave Although we don't know what dark matter is, we do know that it is definitely not antimatter. If it were, it would annihilate with matter, producing extreme amounts of X-rays.  
> **username_1**: @DaveTheWave Well yes, you can conjecture any teapot you'd like, but that's not science. An extra property or charge would be incompatible with virtually all observations and experimental results within particle physics. Antimatter is not dark: It emits light just like matter, and if matter and antimatter meet, it annihilates and emits X-rays. If you were to place a galaxy of matter, in a halo of antimatter, it would be the most spectacular firework ever :)  

---

