in 1995 physicist Ed Witten proposed the existence
of an 11th dimension

he did so to in order to account for a troubling
anomaly in String Theory: five different and
seemingly valid string theories had emerged,
and this had theoretical physics spinning its
wheels

Witten's extra dimension triggered a revolution in
thinking about nature of the universe; for example,
it allowed for the existence multi-dimensional
membranes

these membranes, with sufficient energy,
could be as big a universe

according to M-theory, our universe resides in
one such membrane, and further, it may exist
side by side with other universes that occupy
other membranes in a higher dimensional space

the video below presents these propositions in
greater depth



the above video is a brief excerpt from NOVA's
The Elegant Universe, it can be seen in full here