Any question that has to do with “before the big bang” or “outside of our universe” (those two are the same thing) has the same answer: I don’t know and neither does anybody else.
Everything anyone says about “before” or “outside” is necessarily speculation because we don’t know anything about that and we might never know anything about it.
Now, not all speculation is equal. We can speculate on the basis of beliefs that have no grounding in reason or testable evidence (eg. religion) or we can speculate starting from that which is strongly supported by evidence and reason.
However, even if our speculation starts from a sound, up-to-date, scientific understanding of our space-time continuum, we have no way of knowing to what extent, if any, the physics of “the universe” we inhabit is similar to that which may or may not exist outside of it.
By the way, this renders the Kalam Cosmological Argument, and all other First Cause arguments for theism, completely impotent.
Sorry, William Lane Craig, but your best-known defence of god-belief is, quite obviously, a non-starter.