What did Democritus contribute to our modern understanding of matter?
Democritus is an Ancient Greek pre-Socratic philosopher primarily remembered today for his formulation of an atomic theory of the universe.
According to Democritus' atomic theory, everything is made up of atoms, which are physically indivisible: atoms are indestructible, eternal and invisible, small and unable to be diminished. He also stated that atoms occupy space and differ in size, shape, magnitude, position and arrangement. In his model, atoms are homogeneous.
Using this as a basis to the physical world, Democritus was able to explain all changes in the world as changes in motion of atoms, or changes in the way the atoms were packed together. The theory explained physics and combined mathematics, since its structure was quantitative and subject to mathematical laws.
In his theory, he explained that by atoms moving randomly and colliding to form larger bodies was how the universe began. According to Democritus, the world always existed, will forever exist and is filled with atoms moving about randomly.