Graviton
From the Simple English Wikipedia, the free encyclopedia that anyone can change
The graviton is a little thing that might exist which causes gravity. No one has ever found one. If someone found one, it could lead to a theory which would join together the four main physical forces: electromagnetism, gravity, the strong nuclear force and the weak nuclear force.
As the graviton is a little thing that carries force (gravity), it is classed as a 'boson'. Other bosons include the photon, the gluon and the W and Z particles.