What are quarks and gluons?

For close to three decades, nucleons (protons and neutrons) were considered to be the ultimate subatomic particles. As scientists delved deeper, they detected shadows of yet another layer of matter inside nucleons. Unable so far to crack open a nucleon and bring out one of these shadowy objects for observation and measurement, they named them quarks. It is believed that one up and one down quark make up the proton and neutron. Quarks are held together by a force dubbed the chromo force, represented by particles named gluons, which are just as unseen as quarks.