Dark ages is the period from which very very little written record remains. It absolutely did exist - there's very few texts from between ~500AD and ~900AD in Western Europe, because education system, an integral part of Roman society, was destroyed, and literacy became almost nonexistent, isolated to monasteries where mostly religious texts were created (or more frequently, just simply copied). In some cases, Dark Ages ended with some ruler who forced all his officials to learn to read and write (Alfred the Great for instance, so we have 10x more texts from 10th century England than 9th century - and 900AD was the definite end of Dark Ages there).
It's not about "life sucked" but about "life we know very little about".
And, it's not because of systematic destruction of texts. Systematic destruction of classic era texts was rampant in 5th-7th centuries because they were considered pagan, and thus sinful. And yet, plenty of them remains because of how many of them were created. There was a lot less burning of say, 8th century manuscripts - there was very little motivation for anyone to do that. If few of them remain it's simply because few were ever written.