ok. a parallel universe is a universe just like this one but a lil bit diffrent. there is a copy of everything of here over there. but just a lil diffrent. its all about vibrations.. everything has an unique vibration (very small). the two universes are at the same spot at the same time but they are vibrating on a diffrent frequence. sometimes (rarely) the frequence equals and you get a deja vu (a look in the other side). just for a small moment. ^_^

OR you reason that the universe is infenete and that there is no other way than everything repeats infenete. /done