The universe… which might actually be the inside of a giant black hole. Turns out the observable universe contains more than enough mass to create a black hole the size of the observable universe. This would also make sense because the Big Bang started as a singularity of infinite density, and what’s at the center of a black hole? A singularity of infinite density. This also would jive with the recent developments around the holographic universe theory because black holes have been shown to store all contained information holographically (basically all of the information is encoded on the 2d surface of the event horizon).
The theory that our universe is inside a black hole isn’t new, nor is it accepted as fact by the whole scientific community, but it is a genuine possibility. Then begs the question, is there another universe outside this black hole, and are there universes inside the black holes in our universe? It might be an infinite self contained loop of infinite universes.
Please note that all of this is way oversimplified.