You might want to have a look at this book by a well-known physicist who is one of many attempting to answer these questions: Warped Passages: Unraveling the Mysteries of the Universe's Hidden Dimensions, Lisa Randall 2006

I would look up David Deutsch and M-theory. Both have parallel universes or multiverses, call them what you will. The multiverse theories, or something similar at least, have increased in popularity a lot recently. It seems to be the prevailing opinion now. But there is still a lot of debate over quantum mechanics many different interpretations.

1) "A multiverse (or meta-universe) is the hypothetical set of multiple possible universes (including our universe) that together comprise all of reality. The different universes within a multiverse are sometimes called parallel universes. The structure of the multiverse, the nature of each universe within it and the relationship between the various constituent universes, depend on the specific multiverse hypothesis considered. Multiverses have been hypothesized in cosmology, physics, astronomy, philosophy, theology, and fiction, particularly in science fiction and fantasy. The specific term "multiverse," which was coined by William James, was popularized by science fiction author Michael Moorcock. In these contexts, parallel universes are also called "alternative universes," "quantum universes," "parallel worlds," "alternate realities," "alternative timelines," etc. The possibility of many universes raises various scientific and philosophical questions." Source and further information: http://en.wikipedia.org/wiki/Multiverse 2) "The many-worlds interpretation or MWI (also known as relative state formulation, theory of the universal wavefunction, many-universes interpretation or many worlds), is an interpretation of quantum mechanics. Many-worlds denies the objective reality of wavefunction collapse. Many-worlds then explains the subjective appearance of wavefunction collapse with the mechanism of quantum decoherence. Consequently, many-worlds claims this resolves all the "paradoxes" of quantum theory since every possible outcome to every event defines or exists in its own "history" or "world". In layman's terms, this means that there are an infinite number of universes and that everything that could possibly happen in our universe (but doesn't) does happen in another. Proponents argue that MWI reconciles how we can perceive non-deterministic events (such as the random decay of a radioactive atom) with the deterministic equations of quantum physics. Prior to many worlds this had been viewed as a single "world-line". Many-worlds rather views it as a many-branched tree where every possible branch of history is realised." Source and further information: http://en.wikipedia.org/wiki/Many-worlds_interpretation