According to https://phys.org/news/2016-09-cold-black-holes.html https://phys.org/news/2016-09-cold-black-holes.html a black-hole greater than the mass of the m

According to https://phys.org/news/2016-09-cold-black-holes.html https://phys.org/news/2016-09-cold-black-holes.html a black-hole greater than the mass of the moon would have an apparent temperature less than that of the cosmic background radiation, so it wouldn't have started shrinking yet. And it'd be very hard to see. Temperature is inversely proportional to mass ( https://en.wikipedia.org/wiki/Hawking_radiation https://en.wikipedia.org/wiki/Hawking_radiation ) so heavier black holes are even colder. According to https://what-if.xkcd.com/129/ https://what-if.xkcd.com/129/ a moon-mass black hole would be as big as a grain of sand. I don't know how far out LIGO can detect small-mass black-hole mergers, nor how likely these mergers would be. Nor do I know how detectable micro-lensing events would be.