The Mystery of the Missing Black Holes

In the census of black holes, there are basically two populations: The big, supermassive bruisers that churn away in the hearts of large galaxies, and the smaller, comparatively runty black holes that form when massive stars collapse and die.

But what about the medium-size black holes – the middleweights, the golden retrievers, the four-door sedans – that are neither galactic drain nor single stellar corpse?

According to some astronomers, these intermediate black holes, with masses equivalent to anywhere between 100 and 100,000 suns, should be everywhere. After all, those million-solar-mass galactic drains aren’t just born that way.

Trouble is, the middleweights are mostly missing. Decades of searching have only yielded a handful of strong candidates. Even this week’s episode of Cosmos:

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