There are some theories, and yes, they are all disturbing.
Black holes are space's boogeymen. Although there are a lot of monsters out there in the universe – supernovas, asteroids, death stars – they are never quite as scary. Black holes retain a aura of mystery and menace. How could they not? They retain everything.
For those who have stumbled on this site having never seen, read, or even heard about science fiction, here is a brief refresher course in spaceology. Black holes are what happen when there is too much of a good thing, only without the good. Too many things, too much mass, and the too much gravity that they produce when pushed together in one place, will eventually collapse into a single point with infinite density.
‘A single point,' you say. ‘Is that what makes them take up at least fifty percent of an IMAX movie screen? The fact that they're a single point?'
I don't appreciate your tone, but no. When we see black holes in space operas and documentaries, usually accompanied by a threatening chorus of cellos, we aren't seeing the black holes themselves. We're seeing the event horizon. A black hole's gravitational force is so intense that once objects get within a certain distance of them, they're sucked in. This happens to every massive object, including light. Since we see reflected light, and no light can get to us once it travels within a certain distance, all we see a large patch of darkness.
Obviously, this makes black holes tough to study. So what happens when two big patches of darkness get so close together that they fall into one another?
Sometimes they can end up taking a honeymoon trip together. Just because two celestial bodies are black holes doesn't mean that they are evenly matched. Sometimes black holes will come at each other slightly off center, or the masses of them are unequal. That can cause a movement one way or another. Black holes also give off gravitational waves. There are actual waves in space-time caused by fluctuations in the black hole and the event horizon. Gravitational waves carry momentum, so emitting them unevenly will pushe them one way and push the black holes another.
As a result, the newly-joined black holes, with their newly-expanded event-horizon, can start traveling, sometimes all the way to another galaxy.
Kind of makes you want to check to see that there aren't any black holes around here, doesn't it?
(There's one right behind you.)
Via: Physics Central.