Where did Darius the Great expand the Persian Empire?

Answer 1

By the time Cyrus took over, the Persian Empire was already well established by his predecessors Cyrus the Great and Cambyses. Darius added Thrace, Macedonia and the west of India up to the Indus River.

His major expedition to capture Scythia failed - he was driven back and rescued by the Ionian Greek fleet which had guarded the crossing of the Danube River for him.

His plan to incorporate mainland Greece was forestalled by his having to put down a revolt in Egypt, and he died before resurrecting that plan.