It's been a long time since my basic history classes, but here goes. Middle Ages as I recall was broken up into Early, Late, and High. Roughly speaking, Early went from around 500 to about 800/900 AD. Late went from about 800/900 to about 1200/1300 and the High Middle Ages covered roughly 1300/1400 to about the mid 1500s.
The Dark Ages is a fairly narrow time band that gets its name from the low quantity of written material. As I recall, it went from the 'fall' of the Western Roman Empire to maybe 600 AD. I might be wrong here, but it seems as though the 'Dark Ages' refers primarily to British history; e.g., from the withdrawl of the Romans (early 400s) to roughly 600/700.
Gary