It used to be there was a firm popular image of a black hole.
Okay, so we have an object with an escape velocity so high nothing, not even light, can escape. All it can do is become denser and denser...oh wait, its already infinitely dense. So it's what? Just pulling matter out of the universe?
I think logic says that the old concept of a black hole doesn't work. Then Stephen Hawking rolled up in his wheelchair and went 'Ahem. Thing is? They're not black.'
And got the radiation released by the space immediately around a black hole named after him. Now, we've come to realize that black holes form anchors for galaxies.
The more we learn about these mysterious phenomena, the clearer it becomes that far from being sinks of matter and energy, black holes are a key part of the dynamism of the universe and perhaps, ultimately, part of the key to life itself.
Many, many questions remain to be answered, but this article from National Geographic talks about how supermassive black holes affect the formation of not just the galaxy they are in, but its neighbors.