A Theory of everything or TOE is a theory in physics which unifies the four fundamental forces of nature: gravity, the strong nuclear force, the weak nuclear force, and electromagnetism or the electromagnetic force. The term is used to indicate scope of such a theory, not the actual content of it. There have been numerous theories of everything proposed by theoretical physicists over the last century, but as yet none has been able to stand up to experimental scrutiny. Popular candidates for a theory of everything at the moment include string theory and M-theory.
Theories of everything must be distinguished from grand unified theories (or GUTs), which attempt to unite all the fundamental forces except gravity. We already have a successful theory that unites the electromagnetic and weak nuclear forces into a single electroweak force; GUTs attempt to unify the strong nuclear and electroweak forces.
Many GUTs predict proton decay; however proton decay has not been observed.
Other attempts have included Kaluza-Klein theory, an early attempt to unify electromagnetism with gravity