What lies... beyond the universe?
The universe is much larger than what physicists can observe – perhaps even infinite.
But even if the universe is not infinite, nothing exists beyond its borders, as in that case, the universe bends into itself, so to speak. The bend means that a traveller would in principle return to his starting point, if he continued long enough in the same direction.
Perhaps our universe is not the only one. If so, the universe will be surrounded by other universes – possibly an infinite number in a multiverse. The idea of a multiverse is purely theoretical, and the existence of other universes can probably not be proved.
Outside our universe, there could be a multiverse, which might include an infinite number of universes.