A so-called “theory of everything” which combines the General Theory of Relativity (the theory of the very large, which describes one of the fundamental forces of nature, gravity) with quantum theory (the theory of the very small, which describes the other three fundamental forces, electromagnetism, the weak nuclear force and the strong nuclear force) into a unified theory. However, even the most promising candidates, like superstring theory and loop quantum gravity, still need to overcome major formal and conceptual problems, and this is still very much a work in progress.