J.J. Thompson and the First Subatomic Particle. The first discovery of a subatomic particle would come in 1897, when J.J. Thompson first discovered evidence for the existence of the electron. In the beginning of the nineteenth century, chemist John Dalton had, in the very first workable atomic theory, declared one of the fundamental principles of atoms – those tiny little particles that make up all matter in the universe – to be the fact that they were, by definition, indivisible. Before the end of that century, however, even before the entire scientific community had been convinced of the existence of these little things, the nature of atoms as the fundamental building blocks of matter was already being called into question. Atoms, it seemed, were made up of even smaller things.