I wondered the same thing! This article seems to do a fine job on talking about black hole collisions: http://www.universetoday.com/13002/what-happens-when-supe

I wondered the same thing! This article seems to do a fine job on talking about black hole collisions: http://www.universetoday.com/13002/what-happens-when-supermassive-black-holes-collide/ http://www.universetoday.com/13002/what-happens-when-superma... TL;DR: blackhole mergers are really rare, because one often kicks the other out.