According to the standard model of particle physics, the fundamental forces (strong, weak, electromagnetic and gravity) are predicted to occur as a result of an exchange between particles via "force carrying particles". Also, neutrons and protons are made up of tinier particles called quarks. And it is the quarks that exchange force carrying particles between each other to give rise to the strong force. The force carrying particles are called gluons. Fundamental particle responsible for keeping the nucleus together is muon or meson.