The graviton is a hypothetical massless particle conjectured to be responsible for mediating the gravitational force. The graviton is postulated in part due to the significant success of the Standard Model of particle physics, which has demonstrated that all other fundamental forces are mediated by elementary particles. It would be a spin 2 boson, if the predictions of quantum field theory are correct.
The graviton is, as yet, undiscovered. This is not for a lack of trying, however the gravitational force is significantly weaker than all other fundamental forces, and this presents several technical challenges to detecting it.