There are a number of hypotheses regarding the topology of the universe, each depends on assumptions that we lack the capacity to substantiate, due either to technological restraints or the basic impossibility of particular observations. Based on what I've read of them, and I don't even understand most (read: all) of the hyperdimensional mathematics, everything everyone has posted here is completely wrong, excpet for Nate who is almost comepletely wrong.
One of my favourites in the zero-energy universe, in which the sum total of all the energy of the universe is precisely zero, which neatly wraps up quite alot of problems about where is came from.
Anyway, here is a punctured torus beging turned inside-out.
Don't ask, I cannot answer.