"indeed, these were some of the darkest/harshest time in our recorded history"
I'm still inclined to believe that this is just nowadays optics. The life was very harsh in general for pretty much all but recent history. There were a lot of life risk vectors all around and the capacity to do something about that was modest at most. For us looking back only the major events stand out -- the pandemics like the black death, the major depopulating military campaigns like that of the Mongols, or the climate altering events. People died of diseases, wars, famine, or whatnot all the time though. Not just a few here and there like we see nowadays, but community-wide wipe-outs, with survivors having no-one-they-knew left alive. I doubt that for them it made much difference that the faced calamities were limited only to their region or were world spanning, or that the cause for the latest bane was this or that out-of-control event.
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.
I'm still inclined to believe that this is just nowadays optics. The life was very harsh in general for pretty much all but recent history. There were a lot of life risk vectors all around and the capacity to do something about that was modest at most. For us looking back only the major events stand out -- the pandemics like the black death, the major depopulating military campaigns like that of the Mongols, or the climate altering events. People died of diseases, wars, famine, or whatnot all the time though. Not just a few here and there like we see nowadays, but community-wide wipe-outs, with survivors having no-one-they-knew left alive. I doubt that for them it made much difference that the faced calamities were limited only to their region or were world spanning, or that the cause for the latest bane was this or that out-of-control event.