The Standard Model is the best working theory physicists have for what the universe is made of at its smallest scale. It catalogs every known fundamental particle, quarks, leptons, and force carrying bosons, and describes how they interact through the electromagnetic, weak, and strong forces.
It has been tested to extraordinary precision since it was pieced together in the 1970s, correctly predicting the existence of particles like the top quark and the Higgs boson years before they were found in experiments.
Despite its success, the Standard Model has known gaps. It does not include gravity, it does not explain dark matter or dark energy, and it cannot say why neutrinos have the tiny mass they do. Finding physics beyond the Standard Model is one of the central goals of particle physics today.