Atheism is the belief that there is no god, it is not the belief in the big-bang theory.
It simply comes down to respecting the fact that we don't know a lot of things and that religion isn't the correct answer.
OT: Knowing what happened before the big bang would be nice, what created it and such. But you are really asking a lot to know what happened before, especially given its all non-profit and simply knowing what happened will still link the question "what happened before" again, because people are dumb. In essence, sure it might have been some cosmic being or just some random event, but that has nothing to do with religion and the god's created from religion. Religion has proven itself to be false within even the simplest of logic, lack of factual proof, and silly customs.
In summary: Atheism is simply the belief there is no god. Nothing more, nothing less.