Why do atomic nuclei exist at all? A large nucleus contains many protons in close proximity. Why do these protons, all with the same electrical charge, not mutually repel? ‘Powerful forces’ shows the answer: a strong attractive force that acts between neutrons and protons when they are in contact with one another. Further studies of atomic structure have revealed that protons and neutrons are not fundamental particles. They consist of smaller particles: pions, which are made up of quarks that possess a ‘colour’ charge. The relativistic quantum theory of colour, known as quantum chromo-dynamics (QCD), is described along with ‘quark–gluon plasma’.