It is called Quantum Mechanics, once you make a decision, the universe "splits," into two universes (or more), in each universe, you made a different choice, for G-d knows how long, our universe was splitting into indefinite amounts of universes per second, each one different, so yes, the multiverse theory is most likely true.
I also think yes, there must be more universes. Whatever set off the big bang for our universe, whether it were quantum fluctuations or something else, if it happenend once i see no reason why it can't happen again and again. I also think that (empty?) space is infinite so if our universe was created within that space, it can happen all the time i would think.