Some theorists believe that we live in what is known as a multiverse, a collection of universes which do not appear to interact with each other. This theory is only one among many used to explain the mysteries of the cosmos, with some people avidly supporting it, while others argue that the multiverse theory doesn't hold water, and that there is only in fact one universe: the one in which we live. The idea of the multiverse is so appealing that the concept has been widely used in science fiction and fantasy.
While some scientists can agree on the multiverse theory, they can't always agree on how the multiverse actually works. There are a number of competing theories to suggest how the multiverse is arranged, and what its properties might be. Some theorists suggest that the multiple possible universes are all the result of cosmic splits made in response to major choices, while others propose the idea that the multiverse is related to the singularity which lies at the heart of black holes. These theories are only two among many, so it's easy to see how complex the multiverse theories can get.
According to most proponents of some form of a multiverse theory, the multiple universes do not and perhaps cannot interact with each other. In the case of parallel universes, which include the same people, locations, and creatures, interaction could cause serious problems, as people encountered their parallel selves. In other instances, universes within a multiverse might be mutually incompatible, so if people attempted to cross over, they would be unable to do so, or they would die once they arrived.
Researchers who support the multiverse theory put forward a number of arguments to support it, many of which are drawn from quantum physics, a branch of physics which is designed to deal with complex theoretical material. Equally valid critiques of the theory are also drawn from this branch of physics, illustrating the many ways in which knowledge can be used, in this universe, at least.
Authors sometimes find the concept of the multiverse immensely appealing. In science fiction and fantasy, a multiverse can add to the air of the fantastic, allowing the characters to explore totally alien or eerily familiar worlds as the story moves on. Typically, novelists ignore the generally agreed upon idea that the universes within a multiverse do not interact with each other, in favor of using crossings between universes as a plot device.