A theory of subatomic particles and their interactions. The standard model is a kind of quantum field theory and states that all matter consists of three types of particles: leptons, quarks, and the gauge bosons (gluons, intermediate vector bosons, and photons),
which are responsible for the strong, weak, and electromagnetic forces, along with the more controversial Higgs boson. The standard model was first formulated in the 1970s and was tested repeatedly in the 1980s. It correctly predicted the existence and properties of W and Z bosons. It does not, however, unify all forces, since it does not include an explanation of gravity; a boson called the graviton that might be the mediator of gravity has not been found and cannot be accounted for in the model.