The big bang theory was never a theory to try and explain everything. It was proposed as a result of the observation of the expanding universe. It could never explain what happened before the big band. The multiple universe theory is a higher level concept that says That the big bang and expansion of the universe could have been caused by two separate universes colliding. It could also explain why gravity is as weak as it is in ours. I did see an interesting documentary about a group of scientists who found a quasar in an arm of a galaxy that appears to have a very different red shift than the galaxy it is attached to, throwing into question the expanding universe model and thus the big bang theory. They were saying that new galaxies were born out of the center of older ones. I am sorry but I don’t remember the title of the show. They put forward some compelling evidence though.
Of course the test of the validity of a theory is whether it corresponds to reality and it has to be falsifiable. I don’t know how scientists could detect other universes but I believe anything that exists can be discovered and proven by science eventually.
Until some extra-diminsional beings visits us, I don't think we will ever know what caused the Universe. I don't think our brain pans will evolve enough to sort it all out. And if we did, then what? Do we touch the face of the creator? Or do we find that it all just happened by accident? Sad day for billions. Mass suicide. Me, I'll still treat people with compassion, empathy, and respect for being what we are...the best thing on this rock, shortcomings and all.