I'll answer your question with an analogy. Imagine a really small balloon, so small that it occupies a point. Now, imagine that the balloon is expanding uniformly outward from that point. Note that that central point is not part of the balloon. It's the same idea as to what happened with the BB. In this analogy, the universe is the surface of the balloon. The central point is where the universe started, where the BB happened, but that point is actually not part of the universe (it's not on the surface of the balloon). Incidentally, if you look anywhere on the surface of the balloon (anywhere in the universe), you'll notice that the surrounding area is expanding around the point you're looking at so it looks as though any point is the centre of expansion. That is also true of the universe. If you focus your attention on any point in the universe, it looks as though everything is expanding outward from that point so that it looks as though that point is the centre of expansion.