"The Dark Ages" is a term coined by historians to highlight the lack of documentation of the history England after the withdrawal of Rome in the 5th century. The native Britons were not particularly literate, and the invading Germans were illiterate Pagans, so even church records of baptisms and deaths stopped being kept. Life in the North also got more difficult because of economic contraction due to the cooling climate. Keeping careful records is not high on the list when you're hungry.
A similar situation was happening at the same time in other parts of Europe, and the entire phenomenon was christened a Dark Age.