There is also a theory that states that black holes, since they are singularities - are infact their own universe. It’s also not entirely unrealistic to apply that to our universe being in a black hole itself.

We know the observable universe has an age. In fact, we know there’s a limit to what we can see. We can locate galaxies 32 billion light-years away, but the redshift of its spectra confirms it is still about as old as the universe. Theoretically, just like an object falling in a black hole stretches forever, our expanding universe is the exact same phenomenon. There exist no spacetime paths that allow anything to escape our universe.