A parallel universe has been in man's mind for ages. Is it real? It just depends on your ability to travel between the two universes / demensions. If you can it is real. If you can't it is just an idea.
What do you know about the idea/concept of a parallel universe?
First ... "universe" means there is only one.
So when talking parallels or alternates, it would be "multiverse".
And of course there is *something* outside our "universe" ... Or ours wouldn't be expanding in a "flat" plain. It would expand in a sphere, as any unrestrained explosion does. (at least, that is my belief)
There are more dimensions then you
can concceave of.