It is the smallest fixed particle of mass. I know that somebody will say what bout the quark. But the quark is not a fixed and independent structure. A quark is only a subcomponent of the structure s field. Three XYZ Axis.
Gravity is the constant impelling force associated with mass.
Mass is created by the outward pushing centripetal force and gravity is the impelling force. Gravity is directly proportional to mass. This presents the possibility that a change in the apparent mass will also cause a change in the gravitational effect. To review, a particle with a single axis of rotation is a photon. It has field mass singular rotation, frequency and direction.