The search for a complete and comprehensive theory of everything that unifies all the fundamental forces and particles in the universe, including quantum mechanics and gravity, is a major goal of modern physics. Such a theory is sometimes referred to as a "theory of everything" (TOE) or a "final theory." The current leading candidate for a TOE is string theory, which posits that the universe is composed of tiny, one-dimensional "strings" that vibrate at different frequencies to produce all the particles and forces we observe. String theory also predicts the existence of extra dimensions beyond the familiar four of space and time. However, string theory remains a work in progress and has yet to make any definitive experimental predictions that can be tested. Other approaches to a TOE include loop quantum gravity, which seeks to unify quantum mechanics and gravity without invoking extra dimensions, and various forms of supersymmetry, which postulate the existence of additional particles beyond those in the Standard Model of particle physics. Ultimately, a complete theory of everything would not only explain the workings of the universe at the smallest scales, but also shed light on the origin, evolution, and ultimate fate of the cosmos as a whole.