[btouellette]

The word you're looking for is MACHO (massive compact halo object) and using microlensing observations you can make all kinds of statistical arguments for how likely objects of different masses are to make up the missing matter to match galaxy rotation curves to observations. Basically it doesn't seem likely that there are nearly enough intermediate mass black holes to explain dark matter. Because microlensing is gravitational it doesn't matter what kind of object the MACHO is.

https://en.wikipedia.org/wiki/Massive_compact_halo_object http://iopscience.iop.org/article/10.1086/319636

[spchampion2]

It would be interesting to take the modeled number of MACHO black holes based on lensing studies and compare it to the modeled numbers of intermediate black holes based on LIGO data. I'd love to know if those two numbers substantially differ.

[raattgift]

For several reasons it's more likely that the IMBHs are in clusters like the large Magellanic cloud (LMC) and in some galactic centres rather than far from the centres of galaxies. You can in principle have huge numbers of IMBHs without any of them being MACHOs, HO for halo objects. They can form by having lots of first generation stars collapsing into BHs in open clusters, with resulting many-body interactions knocking BHs into one another (a BBH merger that gives a big linear kick to the resulting BH during balding, for example).

We've looked for signs of black holes in our galaxy's halo (they would gravitationally lense nearby background objects like the LMC and M31 such that we could already pick out the distortions. Our galaxy may be unusual (Sag A* is fairly low-mass, for instance) but the density requirement for MACHO to be a large component of the flat rotation curve of most galaxies (ours being special) would seriously violate the Copernican principle. Moreover, it's really really hard to get a stable galactic evolution when you make MACHOs the sole invisible contributor to rotation curves.

Worst of all, baryon acoustic oscillations appear in the cosmic microwave background more and more clearly as we study it more closely, which effectively rules out large Jupiters/brown dwarfs/neutron stars, and nobody knows how to get enough black holes that survive billions of years when you have to have them already exist between reionization and recombination. But who knows, maybe we'll see a lot of signs of primordial BHs evaporating in halos by (big) surprise.