We observed that everything is moving away from everything else (large scale) so the theory is everything was one in the same place. The discovery of the CMB and the fact that everything is “younger” the farther we look out supports this. Since the expansion would have been everywhere there would have been no center of it, or rather every point is the center of it.

A poor analogy would be if you take a picture on you phone and blow it up where did the center of the expansion begin? It wasn’t in the center or a corner, it happened everywhere at once. So imagine that but start with a point.