In 1954, a young Princeton University doctoral candidate named Hugh Everett III came up with a radical idea: That there exist parallel universes, exactly like our universe. These universes are all related to ours; indeed, they branch off from ours, and our universe is branched off of others. 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.
A parallel Universe is supposedly a Universe like our own. It would develop along a parallel path to our Universe and would therefore be very similar to our Universe. People in our Universe (you, me and almost everyone else) would likely have a counterpart doppelganger in this other Universe.
Modern scientific theories suggest that other Universes do exist. However, only hypothesis and notions exist that suggest these Universes are parallel (similar) to our own. One notion about parallel Universes is that every decision and every possibility for every action in the Universe immediately creates branches of similar Universes. For example, if I flip a coin and it comes up heads, an alternate Universe instantly branches off where my coin comes up tales. This means that all possibilities of an action do actually occur in their own Universe. This notion is fuel for Science Fiction, but I cannot see how such a system could exist.
For a Universe to be similar to our own, it would require some common branch point with our Universe. In other words, our Universe and a parallel Universe would split off from one another. So, here's the problem with parallel Universes magically branching off from one another: Where is all the energy coming from to produce all these infinite parallel Universes? If such a system did occur, there would have to be some infinite energy source to drive it, and some magical mechanism in place to funnel that energy into each Universe. When Universes like our own are created, they consist of a certain amount of energy. For a Universe to split, energy would need to be split between the new split Universes. Of course, one could then argue that perhaps parallel Universes are really just versions of the same Universe, but existing in different states. The problem here is that an infinite amount of energy would still be needed to drive the structure of that system as well since it would still have to create an infinite number of possibilities.
Given current theories and arguments, I don't doubt that other Universes could exist. However, I sincerely doubt that parallel Universes constantly branch off from one another.