Two Crucial Additions To A Decent But Partial Review

1. The microlensing results accumulated over decades, and verified repeatedly, indicate that the Milky Way Galaxy contains on the order of 100,000,000,000 (that is correct: 100 billion) compact MACHO objects. The fact that we can observe these object only by microlensing strongly suggests that they are ultracompact black holes. They are as numerous as the visible stars.
2. Once again, Ethan does not mention primordial black holes although papers on them by noted astrophysicists have been appearing quite frequently at arxiv.org since the LIGO discoveries. PBHs can form in the early phase of the Big Bang, and they can have an almost unlimited range of masses, from far below planetary-mass to many solar masses . Since the MACHOs mentioned in 1. above probably cannot all have been produced in supernovae, the most reasonable explanation is that they are primarily PBHs.

A full and accurate estimate (avoiding the ridiculous delta function assumptions) of the stellar-mass black holes that inhabit our Galaxy and other galaxies leads to the possibility that stellar-mass black holes could possibly comprise all of the enigmatic dark matter.

Let’s not underestimate nature.

Fractal Cosmology