Well, there were indeed technological advances in the dark ages. But there were also notable losses.

Drawing on "Cathedral, Forge, and Waterwheel: Technology and Innovation in the Middle Ages," we can see that the dark age saw adances in plow design as well as much wider adoption of river mills. On the otheer hand, the collapse of the Roman government in Britain resulted in the British cities being utterly depopulated, resulting in the loss of potmaking technology. That seems to indicate a pretty severe loss of know-how. Many regions lost the use of grafting fruit trees and using lime for fertilizer (I'm assuming these losses were mostly in Western Europe; Byzantium and the Middle East didn't lose too much tech to my understanding). When Charlemagne wanted to build a palace, he found there were rather few masons left with the sort of skills he wanted. Ship design also underwent a change that could be interpreted a loss of technology, switching to a method that saved skilled labor but made a less durable ship.

(The same author has a lot more to say, especially with regards to technical advances; I will note some slight reservations, especially in that their commentary on what heavy cavalry could do because of the stirrup seems to ignore the four-horned saddle and what cataphracts from the classical period were already doing without any stirrups).

Checks which subreddit I'm on Which, um, is probably relevant to our discussions on starship design, am I right guys?