Comment by ziddoap
This is very close to an idea known as "Black hole cosmology" -- basically the idea being that the visible universe is inside a black hole, leading to a sort of "nested multiverse".
https://en.wikipedia.org/wiki/Black-hole_cosmology
A related theory, rather than being inside a black hole, is that the other side of a black hole is a "white hole". As matter collapses into a black hole, it is emitted from the white hole, creating another universe.
Here's an article from 2010 that expands on the idea, though this is definitely not the first time (or last time) it was discussed, it just happens to be an easily searchable article.
https://www.space.com/8293-universe-born-black-hole-theory.h...
I'm sure it's not practical, but I always thought it would be interesting if instead of living "inside" a black hole, the visible universe was simply being consumed by a black hole so large it just encompassed everything outside the visible part. So no nesting, the universe just eventually gets consumed entirely by one black hole.