Well, the dark ages came after the late antiquity where slavery was normal. And it took a few centuries for slavery to die out in European societies, though serfdom remained which wasn't too different. And then serfdom in England formally existed even in XIXth century. I'm not talking about Russia, of course, where it played the same role as slavery in the US south.

EDIT: What I meant - this is more about knowledge and civilization, not good and bad. Also 150 years is too much, but compared to 25 years ago - I think things are worse in many regards.