It seems strange that suddenly throughout history things just seem to ‘stop’ for a period of time. If it was due to war or some other catastrophe, surely other civilisations would have records of the events that happened
As the Roman empire collapsed in Europe, maintaining accurate records of every little thing became a lower priority than fighting Goths or whatever. It’s not that history stopped happening, people just weren’t writing everything down. That may coincide with imperial decline but it’s not like everyone just forgot how to do society for 500 years.
A civilization capable of producing art, literature, history, etc. is a complex thing. It relies on trade and communications networks to acquire and distribute resources. If those networks are disrupted for long enough, the civilization can destabilize and eventually collapse. Once that happens, the effort that was put into the production of those cultural artifacts gets redirected into survival, like finding substitutes for all the resources that are no longer coming in from elsewhere. On top of that, the collapse of a civilization can result in mass migration and it’s hard to produce complex culture when you’re not settled in any one place. It usually takes a few generations for the dust to settle and for new societies to start organizing themselves and begin producing cultural artifacts again.
If we’re talking about Europe post Roman Empire, then it’s overblown. We have decent records of the time, and science and technology were still advancing. Additionally, we have plenty of records from other civilizations around Europe about the time period.
It was civilizations collapsing and civil wars breaking out. The issue is that either other civilizations who are unaffected and could report about it didn’t exist or were too far away. We do have records sometimes, dark age simply means there is a lot less than before.
The causes vary. The most famous example of the dark age around the 5th to 6th century AD were triggered by disease, famine due to climate change, and a cascade of roaming hordes being displaced and searching their luck in a new home.
And of course some people still wrote about those times, but more stuff simply got lost in the chaos. Hard to keep a record of your city being burnt down when the neighbouring city where your script is stored gets sacked and burned down the year after (remember that copying books was a work of years at that time, and noone really cared to pay for that when starvation or plundering was looming, being a writer isn’t really a usefull skill in such situations either)
That dark age I mentioned is actually somewhat well reconstructed, we can track at least the later paths of the different germanic tribes took through the roman empire and how everything collapsed on itself. The further back something is the sparser the records are though, the collapse of old mesopotamia for example didn’t leave any higher civilization to write about them as eye witnesses (and if some knowledge survived it was lost in a later era)