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 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 has a length of only 10-35 meters, 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.
Where do the strings come from? There different versions of superstring theory that explain the way subatomic particles behave. 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).