Black Holes and Baby Universes
Some people think that black holes are baby universes – that when a huge star implodes and becomes a black hole, inside of that black hole it’s exactly the same as a Big Bang and that there is literally no difference at all.
Stephen Hawking wrote of something like this in his book of essays titled Black Holes and Baby Universes and other essays. A similar idea was expanded greatly by Nikodem Poplawski, which went further, to say that the reason we experience time in one direction, is because of how this universe was born from a black hole.
Because every galaxy (as far as we're aware) has a huge black hole at its center, this implies that every galaxy in the universe, is the parent of a baby universe. And there are a lot of galaxies!
Not only that – if there is no difference at all between the universe inside a black hole and the universe outside the black hole, then that universe can also contain galaxies, which in turn can contain black holes, ad infinitum.
The author Stephen Baxter wrote a series of hard sci-fi books in which the goal of a massive engineering project was the creation of a universe that was abundantly full of black holes, in order to escape the inevitable end of the current universe through expansion and eventual “heat death”.
If every galaxy has a black hole, and every black hole is a baby universe containing its own galaxies, then there are infinite universes.