Democritus proposed that atoms made up all substances. that they were uncuttable and made of solid particles that could not be subdivided and the movement of atoms caused changes in matter. He proposed his idea after observing changes in matter. Yet, he was not able to provide the evidence he needed in order to convince the outside world that atoms existed. His research and theory remained unchallenged until the nineteen hundreds, when research by other scientists began to show that atoms were composed of much smaller particles than Democritus had thought.