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Baryons are composite particles made of three quarks, as opposed to mesons, which are composite particles made of one quark and one antiquark. Baryons and mesons are both hadrons, which are particles composed solely of quarks or both quarks and antiquarks. Until a few years ago, it was believed that some experiments showed the existence of pentaquarks – baryons made of four quarks and one antiquark. The particle physics community as a whole did not view their existence as likely by 2006 and, by 2008, considered evidence to be overwhelmingly against the existence of the reported pentaquarks. Since baryons are composed of quarks, they participate in the strong interaction. The most famous baryons are the protons and neutrons that make up most of the mass of the visible matter in the universe. Each baryon has a corresponding antiparticle known as an antibaryon in which quarks are replaced by their corresponding antiquarks. For example, a proton is made of two up quarks and one down quark (diagram pictured), while its corresponding antiparticle, the antiproton, is made of two up antiquarks and one down antiquark. (Full list...)