String theory

In physics, string theory, also called superstring theory, is an attempt to unify all the fundamental forces and particles in one theory by modelling them as tiny vibrating "strings". It is currently the most promising theory to replace quantum mechanics, which has been unable to incorporate gravity into its descriptions of the universe.

One of the consequences of string theory is the possibility of many more dimensions than the standard 4, some variations give as much as 20?. These dimensions are coiled up so small that we cannot experience them.

Further reading:

• The Elegant Universe - Brian Greene