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.
The idea that black holes may be worm holes to other universes isn't new. However, in a recent paper published in the journal Physics Letters B, Indiana University physicist Nikodem Poplawski presents new mathematical models that describe how wormholes are viable alternatives to the "space-time singularities" that Albert Einstein predicted to be at the centers of black holes.
National Geographic article on Poplawski's paper