What are the smallest particles in the Universe?

For many years, the only known subatomic particles were protons, neutrons and electrons. By the 1960s, however, advancements in particle accelerator technology had shown evidence of hundreds of smaller constituent particles. By studying these particles -- all part of what is known in particle physics theory as the standard model -- physicists can try to explain all of the forces and matter existing in the universe.

These smallest particles fall into several main categories, most notably fermions, hadrons and bosons.

Fermions

Fermions are the building-block particles. There are two types of material fermions: quarks, which work to hold the nucleus of an atom together, and leptons, which do not. Fermions can be broken down even further: There are different types of quarks, and for each, an anti-quark. Quarks are found in groupings, but leptons are found alone. Electrons and neutrinos are examples of leptons. Fermions have a half-integer spi.

Hadrons

Hadrons are composite particles made of smaller particles. A proton, for example, is a hadron made from a combination of different quarks. Strong interactions bind the hadrons together and they always have charges, but no color. Protons and neutrons are the most stable hadrons. Hadrons come in two classes: baryons and mesons.

Quarks

Quarks are the fundamental constituents of hadrons and interact via the strong interaction. Quarks are the only known carriers of fractional charge, but because they combine in groups of three (baryons) or in groups of two with anti-quarks (mesons), only integer charge is observed in nature.

Leptons

Leptons do not interact via the strong interaction. Their respective antiparticles are the anti-leptons which are identical except for the fact that they carry the opposite electric charge and lepton number. The antiparticle of the electron is the anti-electron, which is nearly always called positron for historical reasons. There are six leptons in total; the three charged leptons are called electron-like leptons, while the neutral leptons are called neutrinos. Neutrinos are known to oscillate.

Bosons

Bosons are subatomic particles that carry force. They help particles interact with one another without touching, much like the forces of gravity or magnets. Unlike fermions, bosons have integer spin. The Higgs boson is believed to be the tiny particle that likely provides mass to all matter. Yet scientists aren't even sure that the Higgs boson exists.The Higgs boson remains one of the key questions remaining in physics and in wrapping up the Big Bang theory. If scientists can identify and study the particle that gives mass to all others, they can explain how the universe started from a seemingly invisible field.