Another definition gets a little more specific with time periods. Many scholars divide the Middle Ages into three parts: Early, High, and Late. Some go further and split the Early Middle Ages, usually dated from the 400s to the 900s, in two and call the parts the Dark Ages and the Early Middle Ages. Others used to just call the whole thing “the Dark Ages.”
But what makes these parts “dark” while the others aren’t? Part of that is the whole loss of knowledge deal. This was right after the fall of the Roman Empire in the west. Europeans outside of monasteries or the still surviving Eastern Roman Empire generally weren’t too interested in scholarship. What with the breakdown of social and political structures, most people had other things on their minds. It wasn't until the various renaissances that pursuit of knowledge became more widely important again in western Europe.
There’s another reason to call this time period “dark” that I think makes the most sense: a scarcity of sources. There’s very little to shed light on what life was like. Compared to other time periods, there just aren’t that many records for what happened or what life was like. We simply don’t know as much about these four-hundred-or-so years. It’s our lack of knowledge that makes them dark, not theirs.
Or there are always my favourite definitions: “the time before safe interior lighting was widely available” and “the time before the invention of the lightbulb.”
Rijksmuseum, Amsterdam