This may sound something straight out of science fiction, well in fact it is countless of science fiction book, movies and television have used the theme of parallel worlds. But before it was used in science fiction it was a scientific theory.
To put this in prospective we once believed the earth was flat, we once believed the earth was the center of the universe, so why not more universes?
It has only been in the last five years or so that this science fiction has turned into science theory. There has been discovers in space such as the WMAP cold spot. The WMAP is a region of the sky seen in microwaves which is unusually large and cold relative to the expected properties of the cosmic microwave background radiation or CMB. The theory is that this spot could be an imprint of another universe. Many leading physicist do believe in the multiverse although they differ on what exactly is the multiverse.
One theory of the multiverse is called a level I multiverse. It is an open multiverse it is infinite in size all of the universes are connected together, there is no begging or end to this multiverse. In this multiverse that is infinite universes would eventually start repeating witch means our universe, galaxy, solar system, and earth would repeat an infinite number of times. So with this we would exist in an infinite number of universes. In these universes we would be doing all kinds of different things, but in some we will be doing the same thing as we are doing in this universe an exact copy, and in other universe that are like ours we would not exist at all.
One theory that I find fascinating is the black hole theory. When a black hole sucks in all kind of matter from planets to suns to countless galaxies no one is sure were all this matter goes. The theory is the matter comes out of a white hole in a different dimension the multiverse and this massive explosion creates a new universe; also know as the big bang theory.
Yet another theory is the bubble theory. In this theory the multiverse is in a different dimension where all the universes are floating around like soap bubbles and when these bubbles (universes) collide with each other they create a new universe.