When did the Celtic empire start?

The term "Celtic empire" is somewhat misleading, as the Celts were not a unified empire but rather a collection of tribes and cultures spread across Europe. The Celtic culture began to emerge around the 8th century BCE, with its influence peaking between 500 BCE and 300 CE, particularly in areas of Western and Central Europe. The spread of Celtic culture was facilitated by trade and migration, but it never coalesced into a singular political entity.