Graviton

Elementary particle of spin 2 (a boson) that is believed to be responsible for the propagation of the gravitational interaction. Its rest mass is zero and so it moves at the speed of light. If a massless particle did not travel at the speed of light then there would be some frame of reference within which it would be at rest. In such a case, its total energy would be exactly zero and it would violate the principle that everything which exists embodies some form of energy.