Stodolsky, L. (2021). Observational Aspect of Black Hole Dark Matter.
Modern Physics Letters A,36, 2150077. Retrieved from https://publications.mppmu.mpg.de/?action=search&mpi=MPP-2019-244.

Abstract:
Advances in high angular resolution astronomy make it conceivable that black hole dark matter could be detected via angular deviation effects. Assuming the dark matter in the galaxy is made of solar mass black holes, there is a non-trivial probability that a line-of-sight through the galaxy, leads to micro-arcseconds deviations, a value that has been discussed for various astronomical projects. In cosmology the effects are magnified by an increased density at early times and an opening of angles due to redshift. If the dark matter is made of primordial black holes, present at the CMB, random deflections of the CMB photons lead to a limit on the angular resolution, approximately ${3}\times 10^{-7} \sqrt{M/M_\odot}\, rad$, with $M$ the mass of the black holes. Using the resolutions of $\sim 10^{-3} rad$ demonstrated in observations of the ``acoustic peaks '' then implies the limit $(M/M_\odot)\lesssim 10^{7}$. While this large value seems uninteresting, improved resolutions would lead to significant limits or conceivably the discovery of primordial black holes.