So, we first have to understand what it actually means to be 'expanding.' The FLRW Metric describes our universe as homogenous (physics works the same at all points), isotropic (looks basically the same everywhere on a large scale), and expanding. In simplest terms, the further back in time we look, the closer things generally seem. Basically, objects that are not gravitationally bound to one another will get further apart as time progresses (as per the metric).
Now, you seem to harbor a very common misconception in that the Universe itself has a 'center.' This is not the case. There is neither a center nor an edge to the Universe so far as we can tell. Expansion was not a single point moving outward but rather a uniform expansion at all points in the Universe simultaneously. The best way I've seen it explained (in an intuitive sense) is as follows:
Picture an infinite sheet of graph paper in the sense that it has no edge. The lines are getting farther apart from one another, but it is still infinite. If we roll back time far enough, there will have been 0 distance between any 2 polar coordinates, and thus our modern physics breaks down. The lines are not getting larger, really, but they are moving apart from one another. At any given reference frame, lines at twice the distance are moving at twice the velocity.
Hopefully this was of some help to you.