The Theory of Everything is a common term used to refer to the attempt by physicists to reconcile the divergent observations of the four fundamental forces of physics. Attempts to develop such a theory date back to the ancient Greeks, but the greatest fervor to surround it dates back only to the 20th century and the demise of one of the greatest minds in history.
At the time, the Quantum Theory was just being developed and the Special and General Relativity were at their peak, readily observable and testable on an infinite scale. An infinitely large scale, that is. Relativity fails on an infinitely small scale and it is here that Quantum Theory was taking hold. However, one of the most fundamental tenets of Quantum Theory is an element of chance, of probabilities. It was this element of chance that led Albert Einstein to give one of his most enduring quotes, "I, at any rate, am convinced that He does not throw dice."
Albert Einstein, the man who had given the world the theories of Special and General Relativity before the age of 40 spent the last half of his life producing theoretical work of very little worth in vain pursuit of a Theory of Everything because he could not reconcile his beliefs with the reality in front of him.