A modification of the Newtonian theory of gravity that has been used as an alternative to the idea of dark matter to explain why the motion of stars in galaxies is not in accord with the expectations of standard Newtonian theory. It is very difficult to provide a theoretical justification for MOND by deriving it from a more general theory of gravity than Newtonian gravity. It is generally, but not universally, thought that MOND is less good at describing observations on the motions of stars in galaxies than are theories involving the existence of dark matter.