For over two thousand years people have thought about the fundamental particles from which all matter is made, starting with the gradual development of atomic theory. A major breakthrough was made when Rutherford developed the nuclear model for the atom -- wherein most of the mass and all of the positive charge is concentrated in a very small region. By the mid 1930s, the understanding of the fundamental structure of matter seemed almost complete and research turned to determining the composition of protons and neutrons in the atomic nucleus.