DON’T FORGET THE 4TH WAY: PRIMORDIAL BLACK HOLES!
In addition to core collapse formation of black holes in supernova, neutron stars merging to form black holes, and direct collapse of stars into black holes, we also know that it is possible to produce astronomical numbers of primordial black holes in the Big Bang event.
Note also that in 2 of the 3 LIGO gravitational wave events detected so far the spins of the initial binary black holes are misaligned. This might imply that these binaries form by a capture process that requires very large populations of black holes throughout the cosmos. So maybe there’s your dark matter.
Note also that Discrete Scale Relativity predicted in 1987 (Astrophysical Journal 322, 34–36, 1987) that the dark matter is composed of black holes with masses ranging from about 0.2 to 35 solar mass. Microlensing has indicated a large population of black holes in our Galaxy’s halo with a most typical mass of 0.5 solar mass, and LIGO events involve the mergers of black holes with masses ranging from about 8 to 36 solar mass.