In the beginning God created… Genesis 1:1
No matter how you slice it, atheists and speculating agnostics cannot get past the concept theologians call creation exnihilo. In case your Latin is not up to par it simply means created out of nothing.
Think about it. If God were to speak to the questioners, He might just say “Okay, go ahead. Start with absolutely nothing the same way I did.
Put that in your “big bang.