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Pryamvade Nataraja is convinced that dark matter is just trillions of black holes. Primordial black holes. James Webb will try to find out more. Sounds pretty plausible. I think she's right. Why not? No strange particles needed. Already in the sixties the idea was proposed.

One can read in this article that LIGO observations don't rule out PBH as a candidate for dark matter.

Primordial black holes (PBHs) in the mass range (30-100) M⊙ are interesting candidates for dark matter but are tightly constrained by the LIGO merger rate. In deriving these constraints, PBHs were treated as constant Schwarzschild masses. A careful analysis of cosmological black holes however leads to a time-dependent effective mass. This implies stricter conditions for binary formation, so that the binaries formed merge well before LIGO's observations. The observed binaries are those coalescing within galactic halos, at a rate consistent with LIGO data. This reopens the possibility of LIGO mass PBH dark matter.

So they could fit the bill.

Why isn't this idea more popular? What's there that advocates against it?

Qmechanic
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MatterGauge
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There's an upper size limit to the individual black holes in a "all dark matter is black holes" model due to the non-observation of gravitational microlensing by dark matter, which leads most people to think that diffuse weakly interacting particles is more likely than black holes are.

https://en.wikipedia.org/wiki/Massive_compact_halo_object

https://en.wikipedia.org/wiki/Weakly_interacting_massive_particles

Zo the Relativist
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    I recall that the microlensing constraints on black holes as dark matter were weakest for black holes in the range of a few tens of solar masses — which is right where LIGO has started discovering black hole mergers. Do you know about recent literature on these constraints? If so, could you edit some comments into your answer? – rob Mar 01 '22 at 15:51
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    @rob: I remember that the frequency of detected mergers of black holes by LIGO is in line with estimates of the number density of black holes by other methods, so I would doubt that the LIGO data gives anyone room for a dramatic undercount of the number of black holes, but I'll leave that conclusion to come from someone who knows more about the observational data than I do. – Zo the Relativist Mar 01 '22 at 16:03
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    Here is a recent review: https://arxiv.org/abs/2110.02821 – TimRias Mar 01 '22 at 16:10
  • How could you detect a 3 km. hole lightyears away from here? Would it really be seen by micro lensing? – MatterGauge Mar 01 '22 at 17:03
  • @Felicia: Generally, lensing is determined by the mass of the object, and not the size, but the upper limit cited in the wikpedia article is on the order of a lunar mass, which would correspond to a Schwarzschild radius on the order of a millimeter. It would be observed by waiting for the object to pass in front of a star, and waiting for the star to bend. Since stars are so far away, a small change in the path of a light ray would cause a comparatively large change in the apparent angle of the deflected star's position in the sky. – Zo the Relativist Mar 01 '22 at 18:07
  • Is that the minimum beneath which a hole can't be seen? – MatterGauge Mar 01 '22 at 18:18
  • @Felicia, yes, but a millimeter is much less than a km – Zo the Relativist Mar 01 '22 at 18:22
  • but yes, if somehow, the dark matter were a gas of sub-thresshold black holes, it wouldn't be ruled out, but then, where do these holes come from, and why don't they collide and make bigger black holes, esp., in the early universe when they were presumably denser? – Zo the Relativist Mar 01 '22 at 18:25
  • Well, if they are only 1mm the chance they meet is rather small. If they together have a mass of, 5 times our galaxy and are distributed in a halo, they should be rather far apart. I dont know why they havent merged early on. Where they came from? Condensed particles in early universe? – MatterGauge Mar 01 '22 at 18:32