I have read these questions and their answers:

None of those answered what I am asking.

Dark matter:

  1. does not interact with EM waves, so it is not visible directly

  2. creates gravitational effects

Black Hole:

  1. does not let EM waves escape, so it is not directly observable

  2. creates gravitational effects

Based on these facts, there seems to be a similarity between dark matter and black holes.

Could it be, that the regions of space (intra-galaxy clusters) where we found gravitational effects that cannot be explained with normal matter, so where we suspect dark matter to exist, that that region of space in between galaxy clusters is just a region that is full of black holes that would have the same gravitational effects and would not be visible directly?

  • $\begingroup$ I believe you're right that none of the linked questions are a duplicate of this one, but this is a hypothesis with a long history and I'm sure there are questions somewhere on the site which address the issue. Maybe someone else can find some. $\endgroup$ – David Z Apr 30 '18 at 9:44
  • $\begingroup$ Dear David Z, the only thing I can think of is if we can prove that dark matter is non-baryonic, do you maybe know how we can prove that, with experiments? Since it is not directly observable, I do not really get how we could prove that. $\endgroup$ – Árpád Szendrei Apr 30 '18 at 9:46
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    $\begingroup$ Possible duplicate of Why can't dark matter be black holes? $\endgroup$ – knzhou Apr 30 '18 at 9:49
  • $\begingroup$ This is a duplicate of a lot of questions, but I guess they're a bit hard to find. The keyword you need to search for is "MACHOs", the general name for this kind of idea. $\endgroup$ – knzhou Apr 30 '18 at 9:50

I'm citing this to answer your question (and I encourage you to read the whole text):

Since black holes have mass, one hypothesis for dark matter was that it was made up of lots of massive astrophysical compact halo objects, or MACHOs. These would be compact objects that do not emit electromagnetically, such as black holes, dead (non-spinning) neutron stars, or old and cold white dwarfs (sometimes called black dwarfs). If lots of these objects existed in the right distribution in the halos of galaxies, it could explain the observed rotation curves. However, gravitational microlensing observations have mostly ruled out the possibility of MACHOs as the explanation for dark matter. The current leading dark matter candidates are known as weakly interacting massive particles (WIMPs)

"The observed rotation curves" are what indicates that there is a matter that we don't see (more in the article linked above), which we call "dark matter".

It seems that gravitational microlensing measurements have ruled out the possibility for dark matter to be made of black holes. Thus the answer is a no.

Though I can't find a paper that clearly announces it, so if anyone can comment I'll edit it.

Edit: by chance, I just attended a talk by J.C. Bellido on Primordial Black Holes as a candidate for Dark Matter. I am no expert, thus I'm unsure I got all his points right, but my understanding is the following :

  • Stellar black holes are too massive and effectively ruled out as DM candidate by lensing measurements.
  • Massive primordial black holes, formed during the radiation-dominated era, are of large mass range and may account for dark matter. Still, more precise lensing experiment might hint that this is indeed the case or not, in the near future.

Probably useful reference : Massive Primordial Black Holes as Dark Matter and their detection with Gravitational Waves

  • $\begingroup$ What about primordial micro black holes? It seems to me that anything with an event horizon smaller than an atom would not produce microlensing detectable with telescopes. $\endgroup$ – Skyler Apr 30 '18 at 13:38

If black holes contribute to most of the dark matter, then constraints from microlensing observations constrain the typical dark matter black hole to be more massive than about 20 solar masses, while the fact that wide binaries in the galactic halos are undisturbed implies an upper bound of about 100 solar masses. It has been suggested in this paper that the black hole masses in the mergers observed by LIGO might actually be explained by a black hole dominated dark matter model.

Such heavy black hole dark matter is a prediction of certain inflationary models, see e.g. this paper.

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    $\begingroup$ To be clear, you are stating that current observations do not rule out dark matter mainly consisting of 20-100 solar mass black holes, and LIGO observartions may be consistent with this theory, and that there are cosmological models that would explain where those black holes come from? Could you cite the microlensing 20 solar mass floor? $\endgroup$ – Yakk Apr 30 '18 at 13:02

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