Who's to say? It's by definition - the "Big Bang" was the moment when the universe,time and space, came into existence. Neither existed before that. To talk about "before" the Big Bang isn't even correct - there is no "Before".

No, the big bang was an event that we know happened, but we don't know that it's when time, space, or the universe came into existence nor is it defined as such. Big bangs could happen over and over. Don't confused the known event, for the theory that it was the beginning, they are different things.