In a previous thread I strongly argued against it being a particularly grim period. It is difficult to compare Europe at this point with other regions, but from my perspective European culture remained just as vibrant as ever. It was different certainly, but how on earth can we judge its quality and why would we even want to do so? I am far less knowledgeable about technological developments, but again I don't think it is a good way to approach history, since people generally tried to do the best for themselves. We should perhaps instead acknowledge their troubled contexts and recognise that they nonetheless lived their lives to the best of their ability, so why should they be seen as inferior to other people living in more prosperous times? More generally, the vast majority of mankind had lived and died working in the field - from their perspective, it is difficult to see how better or worse their lives would be either under the Roman empire at its heights, the Abbasid caliphate, and China under the Tang dynasty, or in 'Dark Age' Europe, when there was perhaps a flowering of peasant autonomy, as the oppressive Roman state disappeared and gave the countryside a degree of freedom.
I'm not sure it was so much about falling behind the rest of the world, more the actual regression of society following the fall of the Roman Empire, the disappearance of central government, long distance trade and towns, the return to subsistence farming etc etc.
That said, the idea that the dark ages lacked any form of advancement has been robustly challenged in recent years.
However, I am not an expert on this but I wanted to point you to a recent AMA on this forum covering this period which was excellent and very informative on this period.
As a followup for anyone with the knowledge to answer - the Muslim world has been traditionally taught as the cultural and scientific center of the world during this period. If Europe isn't as dumpy as some viewpoints suggest, were the caliphates less vibrant?