1) The only reason we have any basis for asserting there existed "the Big Bang" is the success of our best theory of gravitation (general relativity)From UniverseToday.com

Our understanding of gravity breaks down at both the very small and the very big: at the level of atoms and molecules, gravity just stops working. And we can’t describe the insides of black holes and the moment of the Big Bang without the math completely falling apart.

Perhaps you have an explanation?

2) The math doesn't "break down". More precisely, the math is the

*only*thing that informs us that we should speak of black holes as such or that there was a big bang at all. It is, rather, difficulties combining mathematical structures.

3) Gravity doesn't just stop working at either the very small or very big. There exists certain phenomena that we often adequately describe in terms of a gravitational force which we know is a useful fiction. It has an effective domain of validity as a tool. For those who seek to understand in a consistent manner the nature of the fundamental structures and constituents of reality, gravitational force is simply a guide- it gives us constraints and parameters for progress because any successful theory must be at least as successful as was Newtonian gravitation. General relativity doesn't break down and quantum theory even less so. It is simply that we don't know how to unify the two frameworks, and most of the theories that we have of some sort of quantum gravity are ill-formed and all are completely untested (and we don't even know how they might be tested because in many cases all we have is some general idea of what such a theory might be like).