See, multiple universes are like god - there is nothing in the observable universe that would hint at their existence*, unless you're going to baselessly interpret whatever unexplained piece of information you can find, to appear to prove your point of view. And yet some people build their own, personal cosmologies around that hypothesis.
Unless some experimental evidence appears to support the idea, saying that a parallel universe "caused" something is equal to saying "god did it", and similarly rooted in one's religiousness.
Now, take the General Relativity theory - it makes observable predicions, is mathematically coherent, and as of yet not falsified. For as long as it stays so, it's only sensible to accept it's predictions regarding the beginning of the universe, that is: there was not "before" Big Bang, just as there is no "outside" universe - the BB was where(when?) space-time begun, and it makes no sense whatsoever to talk about cases external to it.
Sure, the GR has it's share of problems, but none of them substantial enough to prove it wrong. While some scepticism is(as always) welcome,
(this is where I finally make a subject-related statement:)
when it comes to cosmology, bringing parallel universe in from the area of thought experiments and wild imagination is not scientific, but religious.
*Feel free to prove me wrong by showing some evidence that I've missed appearing recently