Most experienced (amateur or professional) medieval historians these days don’t hold this view at all (and haven’t for a long time). There is, however, a considerable amount of debate about what constitutes “decline” (e.g. Bryan Ward Perkins) vs “transformation” (e.g. Ian Wood), which parts of medieval society are subject to “decline” and why we might see these moments of change not as dark but as different. That conversation is the one currently shaping historians’ language about how the medieval Mediterranean was different in its various post-Roman incarnations, because there’s a considerable amount of variation across the 1,000 years between 500 and 1500. It varies somewhat depending on whether you’re talking about the medieval Mediterranean in the eastern Roman Byzantine half or the western Roman “barbarian” half but it’s still not “dark” in the sense that knowledge ground to a halt.

Knowledge and scientific production were different in the medieval world and somewhat circumscribed, but it wasn’t completely “dark” either. To shorthand some of the fun stuff, we get everything from the Hagia Sophia to the gothic cathedral, Beowulf and the Ring of the Dove, the splitting of white light into a spectrum, the windmill and water mill (huge for refining grain into wheat more easily), illuminated manuscripts in which the Carolinian minuscule prefaces our modern Roman alphabet. I could go on but it’s worth reading Chris Wickham’s “Medieval Europe” survey for some of this (he’s an economic historian, so his work speaks nicely to the dark-age concept of poor and dirty).

In terms of why the dark ages nickname ever existed, there’s a good explanation, with some extra reading, at https://www.reddit.com/r/AskHistorians/comments/1zw63t/ama_late_antiquityearly_medieval_era_circa_400/cfxq0u2/?utm_source=share&utm_medium=ios_app from u/Ambarenya