The 11th dimension is a characteristic of space-time that has been proposed as a possible answer to questions that arise in superstring theory. The theory of superstrings involves the existence of nine dimensions of space and one dimension of time (a total of 10 dimensions). According to this notion, we observe only three spatial dimensions and one time dimension because the other six spatial dimensions are "curled up" or "compactified."
According to superstring theory, all of the elementary particles in the universe are composed of vibrating, one-dimensional mathematical objects known as strings. The theory does not explicitly state what the strings are made of or where they come from; rather, they are proposed as geometric ideals. Each string length is many times smaller than the diameter of the nucleus of an atom. Any given subatomic particle (or hadron) is made of a string that vibrates and rotates at the speed of light. A particular hadron gets its unique identity from the manner in which the string rotates and vibrates according to the dynamics of Einstein's theory of general relativity. The frequency of vibration corresponds to the mass of the particle.

The nagging question remains, "Where do the strings come from?" Also, there are five different versions of superstring theory that explain the way subatomic particles behave. Are all five versions correct, or are some correct and others wrong? In an attempt to answer these questions, some physicists have suggested that there exists an 11th dimension, which is compactified like the other six spatial dimensions we do not directly observe. Superstring theory with the inclusion of the 11th dimension is sometimes called M theory or the theory of everything (TOE).

Rather than being tiny and restrictive, the 11th dimension, which Edward Witten introduced, has a unique quality – it allows a string to be able to expand. An expanded string is known as a 'brane' (short for membrane). How do we visualise a brane? It turns out that this depends on how much energy the string has. Mathematics has shown that the more energy you give a string in the 11th dimension, the larger the brane can become – give it enough energy and it could be the size of the Universe. This begs the question that perhaps the Universe is a brane – are we all living on the skin of a brane?

Thinking about this, there is another logical question to ask – are there other branes? This is a startling prospect since another brane is in fact another Universe. Perhaps there is a brane right next to the one we call our Universe, another brane paralleling our brane – a parallel Universe! The laws of physics and the forces that act within any particular brane are considered to be a manifestation of the strings that are attached to that brane. And different branes may well have different strings and hence difference laws of physics.