I can't resist this one. Whether right or wrong the hypothesis by Nikodem J. Poplatski is so suggestive that I have at least to mention it.
It all has to do with the controversial wormholes (or Einstein-Rosen bridges), thought to exist within black holes. What Poplatski says is that actual observed black holes may be of this kind, hosting wormholes with whole universes inside them and that, therefore, our own universe may well be one of such universes within a black hole. This would provide an explanation for the Big Bang itself, which would be a white hole (something I have always thought quite logical).
The euclidean representation of a wormhole (from Wikipedia)
It's complicated to explain and even to understand. A longer article can be read at Science Daily and the full original article can be found here (pay per view):
Nikodem J. Popławski, Radial motion into an Einstein–Rosen bridge. Physics Letters B, 2010.