A theory that provides a unified description of all known types of elementary particles, all known forces in the universe, and the origin and evolution of the universe. Some believe that superstring theory is potentially a theory of everything. Others believe that it is impossible to formulate a theory of everything and that any such theory could only claim to be a theory for all forces, particles, and observations concerning the evolution of the universe known at the time. It is thought that a quantum field theory or a unified-field theory cannot be a theory of everything. A theory of everything should explain the number of dimensions in the universe and why the number of observable dimensions is four.