Vannak
Here's one idea:
http://phys.org/news189792839.html
That's fascinating, actually, moreso to me because I had actually thought about this before. I had imagined two cases for this: Either it's an infinite myriad of universes spilling into eachother (although the concept of time gets pretty strange here), or because of the amount of matter gradually decreasing as it goes through the black holes, perhaps each universe formed is just substantially smaller than the universe it came from, and as a result, atstral constructs like stars and black holes might be more common in high density universes, but uncommon to nonexistent in lower density universes.
I'm amazed a legit astronomer had similar thoughts. It was just a weird train of thought I had, and I didn't actually place much value in its validity.