Black holes can “eat” each other. A scientist, in an effort to destroy all but the biggest black hole, sends them all traveling toward each other. However, at the last moment, he aborts this process.
The black holes wouldn’t destroy one another, but instead fuse together, making a bigger hole. Since Stephen Hawking states that the surface area of the final hole is bigger than the surface area of the two original holes, he would end up making a monster black hole, capable of destroying entire galaxies.