Everyone talks about the Dark Ages. By that we usually mean the period from the fall of the Western Roman Empire (say 450AD) to the High Middle Ages (1066 if you are English). But professional historians never use this term and haven't done so for years. To them, the Dark Ages are called the Early Middle Ages precisely because they were not very dark.
The term 'dark' originates from the comparative lack of the written sources in the period. Actually, it depends where you are and what sort of stuff you are interested in. In France we have Geoffrey of Tours' History of the Franks and in England there is my own History of the English Church. A good deal of the Dark Ages is much better documented than the second and third centuries AD about which we know absolutely bugger all. Nowadays laypeople tend to see the Dark Ages as dark in the sense of benighted and superstituous, for which we usually blame the church. I've dealt with that particular libel often enough but have been reading Lynn White's Medeival Technology and Social Change and found that the early Middle Ages, especially from 700AD onwards, were actually a period of rapid change.
In war, the stirrup revolutionised the horse in battle and the need for these mounted knights ushered in the feudal system. Meanwhile agriculture became at least twice as productive as it had been under the Romans as the heavy plough, horse collar, horse shoe and three field rotation each improved yield. The result was a population explosion and the bringing in of most European wilderness under the plough. To process all this extra grain technology again came in to play with a rapid spread of the watermill, tidal mill and finally windmill. In terms of development, Europe in 1000AD was streets ahead of ancient Rome. If William the Conqueror had found himself fighting a Roman legion instead of Saxon housecarls, the result would have been no different.