Final answer:
The main idea is that Democritus introduced the concept of atomos, conflicting with Aristotle's four-element theory, and despite being more accurate, Democritus's atomic theory was overshadowed by Aristotle's influence for nearly two thousand years.
Step-by-step explanation:
The main idea of the referenced passage is that ancient Greek philosophers had different theories regarding the fundamental composition of the universe. Specifically, Democritus proposed the concept of atomos as tiny, indivisible solid objects making up all matter, countering the prevailing belief that the universe was a single, unchangeable entity. Other philosophers like Aristotle believed in four elements - earth, air, fire, and water - which opposed the atomic theory. Despite Democritus offering a better explanation of matter’s composition, Aristotle's influence caused his ideas to be more widely accepted until scientific thought around two millennia later realigned with Democritus's view of atoms.