One of my favorite theories about Dark Matter that makes sense with regard to maintaining Einstein's theory is that we've discovered "clouds" of dark matter by observing the gravity it "puts out". Well it's theorized using dimensional theory that these "clouds" could be other galaxies in different dimensions who's gravity is so heavy that it's affecting our own through the Dark Matter. It'd be like being on the underside of a trampoline while someone's standing on it. 
I haven't looked up any new articles about black holes recently, but from what I understand each galaxy has a super massive black hole at it's center. Which most people think that explains how galaxies are able to maintain their elliptical shape, which actually is impossible. It's actually maintained by X, which we substitute for dark matter, lol.