Like part of a cosmic Russian doll, our universe may be nested inside a black hole that is itself part of a larger universe.
In turn, all the black holes found so far in our universe--from the microscopic to the supermassive--may be doorways into alternate realities.
I had a similar thought one time based on a mathematical model I saw while reading a book on string theory. I also considered that each successive black hole could cause a dimension to "collapse". (i.e. our universe would be derived from a black hole in a universe with 4 spatial dimensions) Of course, I am not a physicist but it is interesting to see that they have similar ideas.