Our current understanding of the contents of the universe is based on the Standard Model of particle physics and its extensions (see e.g. Mukhanov (2005), Peter and Uzan (2009)). The Standard Model incorporates the strong, weak and electromagnetic interactions. The hadrons, made of quarks and anti-quarks, feel the strong interaction (and the weak). They are fermionic baryons and bosonic mesons. In cosmology, the key hadrons are the baryonic proton and neutron, but many more hadrons have been detected. Fermionic leptons feel the weak interaction; these include the electron and the three neutrino species. All charged hadrons and leptons feel the electromagnetic interaction. See Table 9.3 for a summary.

This model is able to explain all particles so far observed in colliders and particle detectors, except that experiments have recently detected neutrino oscillations, so that at least two of the neutrinos must have mass. The candidate particles for cold dark matter also cannot be explained within the Standard Model.

This Standard Model allows us to understand the ultra-relativistic early universe, for times t ≳10-10 s and energies E ≲1 TeV. The Large Hadron Collider is beginning to probe E ≳1TeV at the time of writing. One of the outstanding successes of the model is the prediction of light element nucleosynthesis. A brief overview of particle physics in the early universe is given in Section 9.6.