Grand unification,
Grand unified theory or
GUT refers to a
theory in
physics that unifies the
strong interaction and
electroweak interaction. Several such theories have been proposed, but none is currently universally accepted. The (future) theory that will also include
gravity is termed
theory of everything.
GUTs also predict the existence of topological defects such as monopoles, cosmic strings[?], domain walls, and others. None have been observed and their absence is known as the monopole problem in cosmology.