Dark Ages
The Dark Ages is the term used for a period of early European medieval history between the Roman Era and the origins of the current European nation states. The term early medieval is now often used in its place. Opinions differ but this period is sometimes given as between 400CE and 1000CE though now it would most likely be said to be 300-0900AD.
Origins of the Term
The term comes from the fact that so little was written at the time so that it can be hard to see just what actually happened. This is really a Northern European problem and was not overcome until the coming of Christianity brought literacy and, thus, happenings, and folk memories begam to be written down. Having said that it should be noted that Irish monks were still writing events down and, indeed, it was they who spread Christianity to the Northern English Kingdoms. This is why early historians, had little idea of what happened between the withdrawals of the Legions, and the rationalisation of the English [Anglo-Saxon) influx into the Five Kingdoms. The Welsh had stories and some vague writings about Arthur, as did their exiled fellow Britons in Brittany, which were tales of Briton resistance to the English, but the maintales of Arthur are from the High Middle Ages and reflect teh ideas and attitudes of that time. English written history comes mainly from The Anglos-Saxon Chronicle that was ordered to be written by King Alfred and the ASC continued to be written until teh early 1200s. The ASC histories of teh time prior to Alfred's reign are folk history and are of varied accuracy.
The term Dark Ages has fallen out of general use by scholars of the medieval period.
Some say that there is a Dark Age in Ancient Greek history, between the Helladic and the Hellenistic periods. For the layman, this is the time between the great Bronze Age civilizations of Mycenae, Troy and Crete (which likely were the source of the Odyssey-cycle, as popular in the medieval period as today) and the classical Iron Age Hellenes of Athens, Sparta, et al.