Between 1968 and the 2012 discovery of the Higgs boson at CERN, particle physicists assembled what is now called the Standard Model — a quantum field theory describing all known elementary particles and their interactions except gravity. The Standard Model has been tested with extraordinary precision: the electron's anomalous magnetic moment matches the theory's prediction to twelve significant figures, the most precise agreement between theory and experiment in all of science. It is also frustratingly incomplete: it gives no account of dark matter (~25% of the universe), dark energy (~70%), neutrino masses, or gravity. The Standard Model is the most successful and most unsatisfying physical theory ever written.
Matter in the Standard Model is built from twelve fermions in three generations: the first contains the up and down quarks, the electron, and the electron neutrino; the second the charm and strange quarks, the muon, and the muon neutrino; the third the top and bottom quarks, the tau, and the tau neutrino. Quarks combine into baryons (three quarks — protons uud, neutrons udd) and mesons (quark-antiquark pairs); leptons interact only via electroweak forces. Three of the four known forces are gauge interactions mediated by bosons: the electromagnetic force (photon, U(1)), the weak nuclear force (W⁺, W⁻, Z, responsible for beta decay, SU(2)), and the strong nuclear force (eight gluons, hadrons held together, SU(3)). The Higgs field (quantum: the Higgs boson, discovered 2012 at the LHC) gives mass via the Higgs mechanism. The electroweak unification (Glashow-Weinberg-Salam, Nobel 1979) shows electromagnetic and weak forces are one at high energies. Quantum chromodynamics exhibits confinement — quarks are never observed in isolation. The Standard Model has ~20 free parameters (masses, mixing angles, couplings), measured not predicted. What it does not include is conspicuous: gravity, dark matter, dark energy (the cosmological-constant problem: naive SM estimates give a value 10¹²⁰ times the observed), neutrino masses, and the matter-antimatter asymmetry.
The Large Hadron Collider at CERN — built specifically to find the Higgs and probe physics beyond — has confirmed the SM in detail and not yet found anything beyond despite intense searches. Beyond-the-Standard-Model physics (supersymmetry, extra dimensions, grand unified theories, axions, sterile neutrinos) remains heavily theorised but largely unconfirmed. Neutrino physics (oscillations, masses, possible Majorana nature, neutrinoless double-beta decay) is a major frontier. Dark-matter direct-detection experiments (XENON, LUX-ZEPLIN, PandaX) and indirect-detection experiments (Fermi-LAT, IceCube) continue to constrain candidates. Precision tests at the LHC remain sensitive to small deviations that would signal new physics.