The Graviton is a theoretical massless particle that is responsible for gravity.  It must be a force carrier, and therefore a boson.  Scientists predict that it would have to have a spin of 2.  The Graviton is assumed as being a particle because the equations of gravity are similar to the equations of other particle-mediated forces, such as electromagnetism.  Gravity depends on mass, so gravitons must be in a certain amount for any given atom.