Nothing.
[There is, however, a "theory" stating that beyond the Universe (where "beyeond" is understood in a spatial sense) is God. This "theory" is the consequence of an argumentative string: God is the Supreme Being; He is eternal, omnipotent, infinite, omniscient; He created the world solely by his word and by his will; the world is therefore subjected to laws (God's, nature's, aliens' and humans'); so the world isn't free; but God is perfect; so He is also free; so God is exterior to His creation; He is therefore outside the world, beyond the Universe.]
[There is another "theory" which says that beyond the Universe (where "beyond" is understood in a temporal sense) are the Paradise, the Purgatory and the Hell. This "theory" is also the consequence of an argment: the world is created, so it has a beginning; but what has a beginning is not infinite; so the world will perish; but after the world's distruction, there will be the Doomsday, which will establish who was virtuous, who still has to repent and who was definitely vicious; but these people will go to Heaven, to Purgatory and respectively to Hell; so, beyond the Universe, there is something, namely the Hell, the Purgatory and the Paradise.]
PS Both “theories” conceive space and time as separate entities, which suggest a naive vision of the world. Actually the Universe is everything around us, we ourselves and our theories included. Things only exist in relation whith each other, inside of a time-space continuum, because everything that exists, exists at a given time and in a given space. So there is nothing beyond the Universe.

Beyond Universe
PPS This approach ain’t so pessimistic as you might believe. We aren’t damned: for where is nothing, there is no damnation either.


But what about the scientific suppositions that we live just one dimension of the Universe and that there is infinite possibilities for such and such dimensions and we know nothing about them yet because we live a finite space and relative time?
There will be a “einsteinium” time when we’ll have access to all dimensions in the Universe and than the religion and the Darwinist theories we’ll end and a new perspective we’ll be embrace and we’ll know all about us and others?
It’s this only SF?
correction: Is this just SF?
I still believe that what ever is created can never be destroyed, so in this case we recycle our lives after death. You can never destroy something you only change its state of being. I’m pretty sure there are others beyond what we know that are pondering the same thoughts and theories… the truth is we are what we want to believe, our minds are the real universe they never stop expanding with knowledge.