The standard model of particle physics describes the elementary particles (the smallest building blocks of our world) and the forces that act between them: the electromagnetic force, the strong force (that holds together the atomic nucleus), and the weak force that is responsible for certain forms of radioactive decay. The matter building blocks contain the quarks (the particles that make up protons and neutrons) and leptons, which include the electron.
The standard model is the most precise and successful theory ever developed—in some cases, theory and experiment agree out to ten significant digits. The only missing building block of the model is the famous—and, so far, elusive—Higgs particle, which is needed to give mass to all other elementary particles.
Despite its success, most physicists are convinced that the standard model cannot be the ultimate theory. The most apparent problem with the model is that it cannot explain dark matter. Over the last ten years, several experiments established independently that the visible matter (from which humans, the earth, the solar system, and all galaxies are made of) constitute only a small fraction of all matter in the universe. What constitutes the remaining, missing matter—the dark matter—remains one of the great mysteries of modern physics.