According to this article from Scientific American, one of the many implications of recent cosmological observations is that the concept of parallel universes is no mere metaphor. Space appears to be infinite in size. If so, then somewhere out there, everything that is possible becomes real, no matter how improbable it is. Beyond the range of our telescopes are other regions of space that are identical to ours. Those regions are a type of parallel universe. Scientists can even calculate how distant these universes are, on average.
Basically, the author demonstrates that the key question is not whether parallel universes exist but rather how many levels there are. This is a long but very interesting article!
Or if you prefer a short version, you can read a pretty funny "review" of this article on the Boston Globe.