He expanded on the atomic theory proposed by Leucippus, and proposed a the Universe started from a collection of disorganised atoms. These atoms became organised, created the world he lived in and the Universe. He performed many experiments on planets and animals, hypothesised the existence of an absolute space - a void - within which things could move relative to each other, and made contributions to debate on how civilisation, speech and writing might have come about. He discovered that the volume of a cone is one third the base area time the height.
Democritus is said to have written many books, all destroyed by the Romans when the Roman Empire converted to Christianity. We know of his work because he was quoted and referenced by other authors, whose works survived.