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  1. Is there any evidence for dark matter besides just its gravitational effects?

  2. What I’m getting at is...why are we so quick to assume it’s any kind of “matter” at all rather than just some unexplained fluke of gravity?

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    $\begingroup$ Dark matter is matter that interacts only gravitationally, so there could be no other evidence. It is possible that dark matter also interacts weakly, but no such evidence is available yet. There are theories that explain the galaxy rotation without dark matter, but they don't explain gravitational lensing: en.wikipedia.org/wiki/Modified_Newtonian_dynamics $\endgroup$ – safesphere May 30 '18 at 8:10
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    $\begingroup$ The earliest indications of some kind of dark matter go back to the 1930's and in what might be called "modern" form to the late 1960s. I'd hardly describe that as being "so quick to assume". $\endgroup$ – StephenG May 30 '18 at 8:42
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There is substantial evidence (e.g. from measurement of the cosmic microwave background) that the universe is almost flat - that the energy densities in the universe sum up to be almost equal to the critical density.

The contributions due to neutrinos and photons are almost negligble at the present epoch. Dark energy appears to contribute about 70% (which is in concordance with observations of high redshift supernovae). That leaves 30% to be supplied by matter. But when we add up all the contributions of matter that can be detected, we only get to around 1-2% of the critical density. There appears to be lots of matter that we cannot see. Furthermore, we know from big-bang nucleosynthesis calculations and measurements from which the primordial He, D and Li abundances are inferred, that "normal", baryonic matter contributes at most about 5%. Therefore we are left with accounting for 25% of the energy density of the universe by something which cannot be seen and is non-baryonic.

I believe this line of argument is quite separate from the dynamical/gravitational arguments for dark matter, based on the rotation of galaxies, the motion of galaxies within clusters and the formation of structure.

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  • $\begingroup$ It is still an argument based on gravitational dynamics. In this case just on a cosmological scale. However, the fact that a similar amount of dark matter is needed to account for a wide range of gravitational observations is certainly indicative of... something. $\endgroup$ – mmeent May 31 '18 at 6:13
  • $\begingroup$ @mmeent There is an element of truth in that, but you could for example replace dark matter with more dark energy to make the universe flat. Sure, this would violate other constraints, but the flatness of the universe together with the primordial abundances seem to be a separate piece of evidence to me. $\endgroup$ – Rob Jeffries May 31 '18 at 8:18
  • $\begingroup$ Seperate but still purely gravitational. I would also argue that evidence from gravitational lensing is really seperate from evidence from cluster/galaxy dynamics, and both are seperate from evidence from structure formation. All of them however are the form, "we need to add addition matter to our source term for gravitation in order to match observations". I do agree, that the cosmological evidence combined with primordial abundances gives the strongest evidence that we are not just dealing with "dark baryonic matter". $\endgroup$ – mmeent May 31 '18 at 9:47
  • $\begingroup$ If we know that we are missing some portion of 'non-baryonic' matter, it would seem to me that we should include the fermions but I don't see that part of the math/logic. $\endgroup$ – Kelly S. French May 31 '18 at 14:39
  • $\begingroup$ @KellyS.French I presume you meant Leptons? What leptons cannot be seen and aren't accompanied by protons? Only neutrinos, which as I mentioned are thought to make a negligible contribution to the current energy density of the universe. $\endgroup$ – Rob Jeffries May 31 '18 at 19:02
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Short answer: no.

Thus far all evidence for Dark matter is based on gravitational interactions. Be it from galaxy rotation curves, galaxy cluster dynamics, gravitational lensing, or cosmology. This tells us that if it is caused by matter, this matter has little or no interactions with light (or the EM-field in general). That it is matter, is at this point still a hypothesis (however this hypothesis is consistent with a now wide spectrum of observations).

People have also tried to explain the observed phenomena using modifications of graviational physics. The most well-known example is "modified newtonian dynamics" or MOND, but there are other variants as well. These alternative theories typically struggle to explain some observed phenomena such as the "bullet cluster" (an observed collision of two galaxies with a significant displacement of the gravitional mass from the center of the collision, as expect for dark matter halos, but not necessarily for modifications of gravity), or the recent observation of a galaxy without dark matter (which is challenging to explain with a unviersal modification of gravity). Nonetheless, proponents of the alternative theories usually find ways to incorporate these observations.

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  • $\begingroup$ Has anyone calculated how the "bullet cluster" could be explained if the missing mass was in a higher than expected number of black holes? If there were more primordial black holes, how would that change the discrepancy between the theories of Cold Dark Matter vs. MOND? $\endgroup$ – Kelly S. French May 30 '18 at 14:50
  • $\begingroup$ Primordial black holes are simply a form of cold dark matter. $\endgroup$ – mmeent May 31 '18 at 6:14

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