> But that was true, wasn't it? The Dark Ages started when Christianity spread through most of Europe.

No, it is not. As Stryan noted in another response to your comment, the idea that medieval Europe was somehow one uniform culture is incorrect.

I would also add that the term "Dark Ages" is used in different ways by different people. People who don't know much about the Middle Ages often use that term to describe the whole of the Middle Ages, from roughly the fifth century to the end of the fifteenth (and Christianity had already spread around the Roman Empire by the fifth). Others just the early Medieval period (about 500 to 1000). Some limit the term to periods where we just don't have many sources, or it is perceived that we don't (e.g., I've heard it applied to Visigothic Spain).

Fourteenth-century Humanists (who lived at a time often considered to be part of those so called "Dark Ages"!) first used the term to contrast what they thought were the centuries between their lives and the classical period. They even went so far as to emulate the handwriting of the classical texts they favored, thinking they should because that's how the Romans wrote. They didn't realize they were copying eighth- and ninth-century Carolingian hands instead, texts copied by monks and clerics and court scribes because they valued them. (Lower case letters in modern languages that use the Latin characters, like English, are still based on Carolingian minuscule.)

I'm pretty sure most people call them "Dark Ages" because during this time the speed of social and scientific development almost entirely stopped.

And mind you, I'm not saying that it stopped _completely_, but it slowed down to a crawl.