Massless particles will move at the speed of light (photons, gluon, nutrinos (sometimes because they change flavour as they travel, some flavours have mass so m

Massless particles will move at the speed of light (photons, gluon, nutrinos (sometimes because they change flavour as they travel, some flavours have mass so must move slower)) and most relevant to this is the graviton. Graviton is assumed massless because gravitational force appears to have unlimited range (galaxies attract each other over very large distances)