I have to agree with the previous comments that the term Dark Ages is not really a proper historical term. Many great achievements were made during this time and the inhabitants of Europe did not suddenly become retarded and unproductive.

Nonetheless, as far as Anglo-Saxon England is concerned, there are several centuries in which the sources tells us only fragments of what happened. To the historian, things do get a bit murky. And you've got to admit, the Dark Ages is a pretty cool name for a period...