The couldn't afford the electric

"The concept of a Dark Age was created by the Italian scholar Petrarch (Francesco Petrarca) in the 1330s and was originally intended as a sweeping criticism of the character of Late Latin literature. Later historians expanded the term to refer to the transitional period between Classical Roman Antiquity and the High Middle Ages, including not only the lack of Latin literature, but also a lack of contemporary written history, general demographic decline, limited building activity and material cultural achievements in general. Popular culture has further expanded on the term as a vehicle to depict the Middle Ages as a time of backwardness, extending its pejorative use and expanding its scope." http://en.wikipedia.org/wiki/Dark_Ages Interesting.

Possibly because there were so many Knights. (Nights)

To add onto Barcaluv's excellent answer... Historians today use the term "Dark Ages" primarily to refer to periods that are opaque to historical investigation due to a lack of primary sources. Since we have in recent generations unearthed a vast wealth of primary sources from the period 500-1000, Historians no longer regard the period as "dark" - and most of us would love to expunge the term "the Dark Ages" from the popular lexicon, at least in regard to the Early Middle Ages. (We still use it for Greece 1100-800 BC.) In fact, the more we learn about the period and its people, the more we are impressed by the level of intellectual sophistication and activity. It is even now recognized that there was no such thing or period as "The Rennaisance" -- every trait, attitude, and outlook that was thought to be peculiar to it is found to be characteristic of the entire Medieval period. (And while Petrarch thought the quality of their Latin sucked, Petrarch was a purist snob. Every scholar I know who's both a qualified Latinist AND has an ear for great literature and poetry, greatly prefers the flowing and dynamic Medieval Latin to the overblown, pompous, florid, stilted, and even straitjacketed oratory and verse of "the Classics.")