Call

Not to oversimplify this but the very nature of "theory" means that an absolute proof does not exist. In the case of the development of the early universe, it will most likely always remain in the theory domain.

theory simply does the best job of explaining the creation of the universe from its very beginning. All the various explanations of what happened and how it happened are in keeping with the known laws of physics.

A better theory of how the universe got its start and why we are where we are now may arise but until that happens, the big bang theory is simply the best we have.