Is Dark Matter Made of Black Holes

The 2019 Halley lecture n February 2016, the Laser Interferometer Gravitational Wave Observatory (LIGO) announced the discovery of the merger of two black holes, each of which weighed around 30 times the mass of the Sun. Shortly thereafter, it was speculated that these black holes might make up the dark matter that has long been known to exist in galaxies (like our own Milky Way). I will review this possibility and explain why the hypothesis may or may not work.