While scientists are just starting to think about the ways how we could find the miniscule black hole that might be circling the Sun at the edge of the solar system another black hole has been found. What makes it remarkable is that it's the closest one found so far. The scientists who found it said (I kid you not) it's just 1000 light years away.
It's one of those regular sized ones. Not the supermassive kind that can be found at the center of the galaxy or the small, primordial one like the suspected one in our own system.
Such stellar black holes are assumed to be the most common ones. They are the result of stars collapsing into themselves under their own weight after the nuclear reaction fueling them dies out. These black holes are usually small, only tens of kilometers radius while weighting several to several hundred times the mass of our Sun.
The exact physics of how supermassive black holes are formed is unclear, but it is assumed they are the result of accretion of mass around of them or even merging together with other black holes. They can be hundreds of thousands or even millions of times more massive than the Sun.
Smaller than stellar sized black holes shouldn't be able to form by gravitational collapsing. At least in theory. But there can still be smaller black holes in existence. Even as small as few millimeter across. These primordial black holes could have been formed in the immediate aftermatch of the Big Bang when the energies abound were so huge they were able to cram matter into dense enough clump to form gravitational singularities.