Is vacuum a necessity for BigBang? I believe most acknowledged theory for the beginning of Universe states that BigBang happened when nothing existed but vacuum or void. But is there a physical observation or a well-established theory that BigBang can only occur in vacuum where nothing "exists"? Isn't it possible -maybe not probable but possible- that BigBang can occur anytime anywhere in our Universe regardless of its occupants? Could it be that "our" Bigbang occurred in a already-crowded universe? maybe this'd explain the imbalance of matter/antimatter following the primeval state of our universe.