Supermassive black holes
Black holes are the greediest things in the universe. They sit there, swallowing anything and everything including light…and prevent galaxies from falling apart. No, really, the reason galaxies spin is because most if not all of them have a supermassive black hole at the centre; they have a mass that is at least a few million times that of our sun – which is 2×10^30kg.
This article from Physorg does ask what causes them to occur: it was previously thought that they were formed during mergers between galaxies, but this may not be the case after all.