I think there was a dark ages. Since the Roman empire broke down and the dozens of tribes occupied the lands that were once roman, many of the luxuries such as schools weren't paid for due to empires being quite local (which hence made trade difficult by comparison of a large empire with lots of ports and well maintained roads), so things like book learning wasn't as common as it was in the roman era. Henceforth, there are less written sources during the dark ages (I would define it between the fall of rome (476AD) to around about the first crusade (1098)) than during the classical era. This is one of the reason that it is reffered to the dark ages IMO.
just my 2 cents as I am not overly knowledgable on the subject.