[Coda]

Not 10-11 alternate universes. When they say "dimension" they mean geometric dimension, not "parallel dimension". The short version of the theory is that particles are vibrating in directions other than just the three dimensions that we're familiar with, directions that we're not capable of perceiving.

The idea of past/present/future "happening at the same time" is a little bit of a contradiction. If it's at the same time, then it's not a flow of time, so you can't really call it "past" and "future". What the physicists mean when they describe it that way is more like... imagine a movie reel, every moment in the movie is right there on the spool in your hands, one big solid object that exists as a unified whole. I've heard it described as a crystal.

That's not to say that parallel existences are impossible. Nothing about the above rules that out; all I'm trying to do here is pin down the concrete claims so that you understand what it is you're saying. There are some theories known as "configuration space" that describe not only our universe but all possible universes and how they relate to each other.