The fundamental theory behind the structure of atomic nuclei is known as nuclear theory. It describes how protons and neutrons (collectively known as nucleons) are held together in the nucleus by the strong nuclear force, one of the fundamental forces of nature. This theory explains the stability of atomic nuclei and the mechanisms of nuclear reactions, including fission and fusion. Understanding nuclear theory is pivotal in fields such as nuclear physics and has critical applications in energy production, medicine, and understanding stellar processes.