Galaxies are, in real terms, just stars clustered together, marking them out as different from the empty space around them. For a long time, scientists wondered what drew stars together like this in the first place - and now we know it's something sinister. Specifically, it's a supermassive black hole.
Supermassive black holes distinguish themselves from more regular black holes purely by size - Sagittarius A sits at the center of the Milky Way and as such is 'our' black hole and is a million times larger than Earth. Its density is beyond imagination, having the mass of four million Suns. There is one at the center of almost every galaxy we've been able to study in detail, and all of the stars rotate around it - more than likely destined to one day fall in.