Multiverse is a theoretical or mathematical concept by Tenmarg and other scientists. Where all natural laws, physical parameters are different.whats outside of observable universe? What's before big bang?it needs to construct a holistic approach to answer that.
Though multiverse is not a scientifically proven theory till date , but all possible outcomes should exist by quantum mechanics. It's called many world interpretation theory.
Our universe is one of them. There we may go one universe to other universe by time travel. The timelike path may difference all possible universes.