Why was the Atomic Universe Theory by Democritus rejected?

In modern times, that would be because the understanding of a completely indivisible atom with no parts has been disproved with the advent of modern nuclear physics. read more

Electrons as a cloud (capable of being separated from the atom) and nuclear fission (splitting of atomic nuclei) and fusion (merging of atomic nuclei) put Democritus’ model as simplistic, regardless that it was spectacularly insightful for its day. read more

Ultimately, though, Aristotle and Plato, two of the best-known philosophers of Ancient Greece, rejected the theories of Democritus. Aristotle accepted the theory of Empedocles (Empedocles argued that all matter was composed of four elements: fire, air, water, and earth. read more

His ideas were rejected because his beliefs contrasted greatly with those of Aristotle's, whose ideas were widely accepted at the time. Democritus believed that matter was made up of small particles called atoms but he could not explain what held atoms together, resulting in other philosophers rejecting his ideas. read more