pretty much any metric you could come up with to "measure" civilisation -- literacy, urbanisation, economic output, monetary economy -- went to historic lows.
slightly orthogonal to this, but the original motivation behind the 'dark ages' label was that for large parts of europe there are very few written records for the 5-7th centuries. e.g., we know practically nothing what happened in 5th century england because the only written source -- gildas -- is mostly concerned with pontificating about sinful behaviour in artful ways. even some actual people he mentions in passing get biblically coded nicknames so we have to make wild guesses who's he referring to. and that's our only source for pretty much a century.
Also Bede although that was written somewhat later (8th century).
When writing Why the West Rules for Now Ian Morris attempted to quantify the overall level of social development in the Western and Eastern cores in a very detailed way. (It used to be available online as a sort of appendix but I don't immediately see it.) In any case, you see this decline in the West after the fall of the Roman Empire that wasn't reversed for many centuries.
slightly orthogonal to this, but the original motivation behind the 'dark ages' label was that for large parts of europe there are very few written records for the 5-7th centuries. e.g., we know practically nothing what happened in 5th century england because the only written source -- gildas -- is mostly concerned with pontificating about sinful behaviour in artful ways. even some actual people he mentions in passing get biblically coded nicknames so we have to make wild guesses who's he referring to. and that's our only source for pretty much a century.