I have created a hypothesis about the formation of supermassive black holes affecting the formation of galaxies. Feel free to critique but don't steal my idea, please.
Black holes have an affect on galactic formation by bringing in planets and asteroids. The larger gravitational pull of of the black hole, asteroids and planets would then have a larger gravitational pull. This would cause more materials to be attracted towards the black hole. (The black hole would gain more mass when it sucks in matter, so the gravitational pull would still increase.) This would keep snowballing until it turns into a very large black hole classified as a Supermassive black hole. More "stuff" would keep surrounding the black hole that it can't suck in all of it and it would start to create a galaxy. This could also happen in an already formed galaxy and it would just make it larger, or smaller depending on if there is anything close by.