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Dark matter may not be found in primordial black holes after all. After the Laser Interferometer Gravitational-Wave Observatory (LIGO) watched a pair of black holes collide for the first time in 2015, speculation swirled that the black holes might be causing the strange gravitational effects that we attribute to mysterious dark matter. Now, that seems unlikely.
Primordial black holes – those that formed in the seething inferno just moments after the big bang – were proposed in 1971 by Stephen Hawking. Until LIGO there was no evidence for them, but the LIGO black holes had masses that meant they could plausibly be primordial black holes.
If all dark matter took the form of black holes about the size of the ones that LIGO has detected, we ought to see a few black hole collisions a year, which is the frequency