The standard model of particle physics is a theory concerning the electromagnetic, weak and strong nuclear interactions which mediate the dynamics of the known subatomic particles. Developed throughout the early and middle 20th century, the current formulation was finalized in the mid 1970s upon experimental confirmation of the existence of quarks. Since then, discoveries of the bottom quark (1977), the top quark (1995) and the tau neutrino (2000) have given credence to the standard model. Because of its success in explaining a wide variety of experimental results, the standard model is sometimes regarded as a theory of almost everything.
Still, the standard model falls short of being a complete theory of fundamental interactions because it does not incorporate the physics of general relativity, such as gravitation and dark energy. The theory does not contain any viable dark matter particle that possesses all of the required properties deduced from observational cosmology. It also does not correctly account for neutrino oscillations (and their non-zero masses). Although the standard model is theoretically self-consistent, it has several unnatural properties giving rise to puzzles like the strong CP problem and the hierarchy problem.
Nevertheless, the standard model is important to theoretical and experimental particle physicists alike. For theoreticians, the standard model is a paradigm example of a quantum field theory, which exhibits a wide range of physics including spontaneous symmetry breaking, anomalies, non-perturbative behavior, etc. It is used as a basis for building more exotic models which incorporate hypothetical particles, extra dimensions and elaborate symmetries (such as supersymmetry) in an attempt to explain experimental results at variance with the Standard Model such as the existence of dark matter and neutrino oscillations. In turn, the experimenters have incorporated the standard model into simulators to help search for new physics beyond the standard model from relatively uninteresting background.
Recently, the standard model has found applications in other fields besides particle physics such as astrophysics and cosmology, in addition to nuclear physics.