Democritus believed that the universe was composed of atoms, correctly, of course. However, at the time, Plato's and Aristotle's hypothesi were just as valid because everyone was a philosopher. There were no scientists back then.
Democritus postulated that all matter is composed of indivisible particles called atoms. He argued that atoms are part of all matter irrespective of the state of matter. These atoms were indivisible in nature.
The word atom is derived from the Latin word 'atomos' which means indivisible.
