What cavediver said is spot on.
The driving model for the big bang is general relativity. This is one of the most stringently tested models in science, and so far it has passed all tests will flying colors. A consequence of this model is an expanding spacetime; and this too is confirmed by observations. No other model makes sense of what we observe; and no other model is as thoroughly tested.
Ironically, however, we know that general relativity fails at quantum scales. It is a "classical" theory; and a unified physics will need to deal with gravity on quantum scales. It follows that as you approach the singularity there is a region where existing physics fails. But what happens in that region will not be a confirmation of population intuitions. There is a great unknown near the singularity; but if you think of it as not knowing what happened "before" then you underestimate its strangeness. Time and space are no longer anything like what we expect, and the popular associated of a word like "before" will not have their familiar meanings.
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