The theory of relativity explains the gravitational force. Together with the standard model it is the most accepted scientific model of the world. So far, the two contradict each other in a few important points. Scientists are working on new models which inclued them both.

The most important idea in relativity for the standard model is that mass and energy are different aspects of the same thing. This means for example that a particle which is pure energy (a photon ) can turn into other particles which have mass but less energy. These new particles are always created as particle-antiparticle pairs because of the conservation laws. In analogy, when a particle and an antiparticle bump into each other, all their mass is turned into energy, a process we call annihilation.

About Theory of Relativity in German. (Relativitätstheorie)