A mirror universe exactly like ours but opposite.

[SheilaT]

5. Serious physicists

are divided on whether alternate universes actually exist. There's one theory out there that every single small decision anyone makes results in a branching off of another universe, so that new universes are coming into being all the time. Others say, no, it doesn't happen.

And if there really is a multiverse, there's also divided opinion about whether or not we could ever cross over into another one. It seems to me, without actually researching this very much, that the consensus is that there may very well be other universes out there, but we will never know for sure because we will never be able to directly observe or interact with them.

That's why I like science fiction, especially alternate history. That and time travel.