The term originally refers to the idea that it's an age of ignorance by Renaissance scholars essentially having a massive Classical Antiquity appreciation binge. This gets amplified by Protestants as an attack on Catholicism. Some more modern people get even more zealously anti-religious and view it as something pushing back the industrial revolution and other things back by 1500 years or so. This especially gets intertwined with things like unilinear cultural evolution models [1], where the Middle Ages end up getting classified as going backwards in culture. Most of these beliefs are basically complete and total hogwash in terms of actual evidence.
Archaeology can trace a rough outline of the economic history of Europe. We see a peak of economic activity in the first and second centuries AD, a decline in third century, which stabilizes somewhat in the fourth and fifth century, before plunging. It hits its nadir in the eighth century, and then starts recovering, surpassing its pre-collapse highs sometime between 1000-1300, depending on which metric you're looking at. If you want to refer to the time from ~500-1000 as a "Dark Ages" on the basis of poor document preservation and general economic collapse, that's actually a reasonable, grounded definition.
But if you're trying to use the term to indicate a loss of knowledge somehow, well, it just doesn't work. Even during the absolute worst depths of the post-Roman collapse, you would still be speaking Roman, learning from Roman textbooks (if you're fortunate enough to be learning), practicing Roman religion, obeying Roman laws. And there's a decent chance you might be doing that despite your village never having once been part of the Roman Empire.
[1] Or, more simply, Civilization-style tech trees.