Can black holes merge?

Yes, can black holes can merge. When two black holes get quite close, it actually becomes inevitable that they'll end up spiralling in towards each other and merge together. This causes a massive energetic event. What you get at the end of it is just another black hole, but bigger. But what it can also do is form ripples in space. If you know anything about relativity, then you know that gravity is caused by bending of space. When you merge two black holes you get ripples, but so far they're only a theoretical idea and scientists are trying to look for them. If we could spot them, then that would really prove relativity.