Even though the universe is expanding you have to understand two usually accepted concepts. First if the universe might just be finite but unbounded. That is if you were to go in one direction you would eventually arrive at the point where you started. The second thing is that even if the universe is expanding it isn't expanding from one point, Just like the surface
of a ballon doesn't expand from one point. So the universe wrapping around with the universe expanding in every direction from
every direction amounts to no center. However I also heard that the universe spins as well wich also would inply a center and to that I have no answer.