You have to remember that the big-bang wasn't just the creation of matter/energy; it was also an explosion of the dimensions of space and time themselves. The big-bang didn't occur in any particular "location" in space, because the big-bang gave rise to space itself.
Consider a "flatland" analogy: you live in a 2dimensional universe, there is only forward-back, and left-right. You also live on the surface of a balloon. The big bang, is when the balloon first began to expand from an infinitesimal point; at some time later, after the balloon has expanded, you try to figure out where the big bang "happened" i.e. the center of the universe. To do this, you look at a much of markers which are fixed in place, because the universe is expanding, it must be expanding from the 'center'--so where-ever things are expanding FROM is the center. On the surface of the balloon, however, every point is moving away from every other point at the same rate (imagine drawing something on the balloon, and blowing it up--it all stretches out evenly)--which shows that there IS NO center.
Astronomical observations show us that every location in the universe is basically equivalent, that can be interpreted as meaning that either there is no center, or every point in space is the center.