The concept of a parallel (or mirror) universe was first introduced to the world by Hugh Everett III in 1957. Josh Clark at HowStuffWorks.com elaborated in October 2007, "Within these parallel universes, our wars have had different outcomes than the ones we know. Species that are extinct in our universe have evolved and adapted in others. In other universes, we humans may have become extinct...With his 'Many-Worlds' theory, Everett was attempting to answer a rather sticky question related to quantum physics (which) suggested that there are other laws at work in the universe, operating on a deeper level than the one we know."
"Surely, somewhere else out there," Andrew Zimmerman Jones and Daniel Robbins from 'String Theory For Dummies' argued, "are other planets exactly like Earth...We don’t see these other universes because our cosmic vision is limited by the speed of light. Light started traveling at the moment of the big bang, about 13.7 billion years ago, and so we can't see any further than about 13.7 billion light-years.
"If Level 1 ("If you go far enough, you’ll get back home") parallel universes do exist, reaching one is virtually (but not entirely) impossible. For one thing, we wouldn’t know where to look for one because, by definition, a Level 1 parallel universe is so far away that no message can ever get from us to them, or them to us.
"In a Level 2 ("If you go far enough, you’ll fall into wonderland") parallel universe, regions of space are continuing to undergo an inflation phase. Because of the continuing inflationary phase in these universes, space between us and the other universes is literally expanding faster than the speed of light — and they are, therefore, completely unreachable.
"Level 3 ("If you stay where you are, you’ll run into yourself") parallel universes are different from the others posed because they take place in the same space and time as our own universe, but you still have no way to access them. You have never had and will never have contact with any Level 1 or Level 2 universe (we assume), but you're continually in contact with Level 3 universes — every moment of your life, every decision you make, is causing a split of your 'now' self into an infinite number of future selves, all of which are unaware of each other.
"A Level 4 ("Somewhere over the rainbow, there’s a magical land") parallel universe is the strangest place (and most controversial prediction) of all, because it would follow fundamentally different mathematical laws of nature than our universe.
"In short, any universe that physicists can get to work out on paper would exist, based on the mathematical democracy principle: Any universe that is mathematically possible has equal possibility of actually existing."
Josh Clark concluded, "Since Many-Worlds was proven possible, physicists and mathematicians have aimed to investigate the implications of Everett's theory in depth. But the Many-Worlds interpretation is not the only theory that seeks to explain the universe. Nor is it the only one that suggests there are universes parallel to our own. Do parallel universes really exist? According to the Many-Worlds theory, we can't truly be certain, since we cannot be aware of them. The string theory has already been tested at least once -- with negative results. Dr. Michio Kaku still believes parallel dimensions do exist, however."