Advanced string theory(i believe) states:

There are an infinite amount of universes(picture these as bubbles or such floating through nothingness, this isn't accurate most likely but i am just trying to create an illusion)Yet on each universe(i think this is what it said)there are an infinite amount of universes overlapping one another.

I believe that is what it states, and what makes it so wonderous.

basically there are infinite amount of other universes. But at the same time there are an infinite amount of (something not sure of word)on this dimension, imagine this world, but different, and on this world as well. we just can't see it. it's like the idea of different timelines proposed by john titor.