They do collide, sometimes! What you get is a larger black hole, and some gravitational waves – you might have heard about these on the news recently. When the two colliding masses are very large, they spin about each other as they get closer, going faster and faster until they collide. This produces a sort of ripple in space. If it’s large enough we can detect it on Earth with a very sensitive instrument called LIGO (Laser Interferometer Gravitational-Wave Observatory).