The Dark Ages is the term used for a period of early medieval history between the Roman Era and the origins of the current European nation states. The term early medieval is now often used in its place. Opinions differ but this period is sometimes given as between 400CE and 1000CE.
Origins of the Term
It probably originates from British historians, who had no idea of what if anything existed between the withdrawals of the Legions and the point where all the awesome stuff stopped and the medieval bullshit began. They had heard rumours of King Arthur but lacking documentary proof he existed, they dismissed him as quote "Boring British drivel, without a single hint of battleaxe, and with not one single splitting of a head. Bore-ing." Then again, British historians take a lot of ecstasy, mostly because, come on, they're the kinds of people who deal with shit like pagan legend (all, of course, untrue. Oh, wow, you guys troll yourselves, don't you? Cool beans, makes my job a lot easier!) and milleniallist rantings. They had occasional chronicles, which were unsupported by "hard evidence" (which is one of the only sexual jokes British historians make, usually made right before everyone decides to go out for pizza, hookers, and blow).
They therefore constructed the "Dark Ages" as a period where Nothing Of Significance Happened, and people simply worked all day long and sat around in the dark at night, waiting for the glorious sunrise of academia, which was taken, pretty much, to commence with King Alfred, who as well as burning drop scones, set up (or tried to) a library. Holy shit, it's like all of this is trolled for me! (Cnut obviously didn't count -- he did silly things with thrones and tides and anyway, he was Norse.) The historians' definition of Full Daylight probably coincided with Edward the Confessor, since they knew he was supposed to have been a learned man, and since they needed to discuss whether Harold Godwinson had or had not sworn holy oaths to William the Bastard prior to his (Harold's) seizure of the throne on Edward's death, and therefore whether William was a bloody invader and all that noise, though bloody invader sounds cool, let's go with that. All of us reenactors basically just sit around LARPing all day and battling with $2.95 plastic swords we picked up at the costume shop, anything to escape the reality of our pathetic lives in our parents' basements.
The term Dark Ages has fallen out of general use by scholars of the medieval period.
There is a Dark Age in Ancient Greek history, between the Helladic and the Hellenistic periods. For the layman, this is the time between the great Bronze Age civilizations of Mycenae, Troy and Crete (which likely were the source of the Odyssey-cycle, as popular in the medieval period as today) and the classical Iron Age Hellenes of Athens, Sparta, et al. Oh, wait, that's not for the layman at all, is it? That's for the dorks who would piss away precious time researching shit like that instead of getting lulz, such as trolling Cunnan.