Some major celtic details on the origins of indo-european languages

A question still open to debate is the unresolved "Celtic conundrum", i.e., the origins of the Celtic-speaking people. The traditional theory postulates that the compact presence of Celtic languages in the extreme west of Europe was also due to an invasion of Celtic people from Central Europe during the first millennium B.C. However, this theory must face a mass of problematic questions and explain some minor but crucial details