The idea of the “Dark Ages” came from later scholars who were heavily biased toward ancient Rome.
In the years following 476 A.D., various Germanic peoples conquered the former Roman Empire in the West (including Europe and North Africa), shoving aside ancient Roman traditions in favor of their own. The negative view of the so-called “Dark Ages” became popular largely because most of the written records of the time (including St. Jerome and St. Patrick in the fifth century, Gregory of Tours in the sixth and Bede in the eighth) had a strong Rome-centric bias.
