Could dark matter be made of black holes? I have read these questions and their answers:


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*Dark matter black holes

*Could Dark Matter form black hole?

*Does dark matter originate from matter falling into a black hole?

*following up dark matter accretion in supermassive black holes
None of those answered what I am asking.
Dark matter:


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

*creates gravitational effects
Black Hole:


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

*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?
 A: 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 :


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*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
A: 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.
