Fundamental particles which join together in triplets and doublets to form hadrons such as protons and pi mesons . The first theory was developed by Murray Gell-Mann and George Zeweig in 1964. There are six types of quark, known as up, down, strange, charm, truth and beauty. In the present day universe, quarks must exist as triplets or doublets. Only in the early universe did they exist as free particles.
Quarks have an additional quantum property, known as colour.
It can be red, green or blue. Hadrons have to be white, i.e. a combination of a red, green, and blue quark or a colour and its antimatter counterpart. For example, a proton is composed of two up quarks and a down quark in order to arrive at the correct charge. Those quarks must then be red, green and blue. Mesons are composed of two quarks.