a theory has been postulated in a recent issue of New Scientist in which some dudes postulate that what we think of as the Universe is in fact one giant five dimensional black hole.
Admittedly this is pretty difficult as most people (me included) immediately would think of the classic 4-D version of a black hole, in which the enormous tidal forces would instantly pull anything - whether the planet Earth if it fell into a 4-D black hole, or light, or a spaceman adrift - apart. Anyone who'se read Stephen Hawkins would be familiar with his notion that time, light and space would become distorted, and with the example of a spaceman in a ship orbiting a star collapsing into a black hole, where the signal from his mate - whose on the star falling into the black hole - taking longer and longer to reach the guy in the spaceship until it becomes infinate. Thus time is stretched, which has led to exotic theories about black holes leading to gateways into other galaxies, etc.
Anyway, if this new theory is true, it would surely challenge the prevailing theory of black holes, as if it's true then our very existence shows that it's possible to live within a 5-d black hole. Of course, this doesn't take into account that there may be 4-d black holes within a 5-d one - and in fact, it's believed that in the centre of our galaxy (of which we are a tiny part of one spiral arm) there's a black hole. The idea of 4-d black holes within 5-d ones is pretty mental.
It could mean that what we think of as the Universe as a whole may be just one 5-d black hole existing within an unimaginably vaster Universe - one with alternative realities. Hopefully these alternative realities don't contain this pitiful lot.