Is there an end on a Mbius strip? Is there a centre on the surface of a sphere? They are finite so there must be centre, right? Of course the answer is no, however, I understand why you think like this, since the way modern physics and astronomers think of the universe are in abstract mathematic thoughts which we cannot visualize because of our limitation to only see three spatial dimensions and not a fourth. Because I had a project about the fourth-spatial dimension I started to have a pretty good idea of how the fourth dimension is like and therefore I can understand why they, physics and astronomers, think of the universe as with more dimension than just the three we can see as human beings. The best I can do is telling you a lot of analogies, but of course, the problem with analogies are they catch some of the point but also miss some of the point, they are never are never perfect. The most popular, and probably also the best, is to use two dimensional analogies. If you have any kind of interest in such thing I either recommend you to search through the Internet and/or read the book Flatland: A Romance of Many Dimensions by Edwin Abbott Abbott from 1884. Here we can read how flatlanders trying to understand phenomena that does not make sense in a two dimensional world, as they live in, and the idea of a new dimension, that they cannot see but must be there, to explain the phenomena.
In any case; we still do not know the exact shape of the universe, but it is very likely that it has no centre at least.