It covers 200-1200 AD (roughly) with stories from around Europe and the middle-east, highlighting the access, travel, science, and art of the time, in a way that really shows that it was far less "dark ages" than commonly thought.
There really was a steep decline in civilization, cities emptied, many completely, literacy rates in the tank, many many people regressed to subsistence agriculture.
No it wasn’t the whole world, the successful successor states to the Roman Empire didn’t experience it. It was also not complete, but don’t make the naysayers mistake of overstating the contrary evidence.
I read a local book about violin makers. I was shocked at how many there were, in the almost entirely rural community around here. To me, making a violin requires some pretty serious, specialised woodworking skills. But they document hundreds of local farmers - subsistence farmers - apparently having those skills. And they don't even count those who "just made a fiddle or two".
Which isn't to say there can be no such thing as dark ages. When there's so much war and chaos that no one has time to build much of lasting value, whether materially or culturally, that's pretty dark. Yet the few things that come out of such times and places (the English border ballads comes to mind, or the hymns of Paul Gerhardt) can be all the more beautiful.
Acknowledging the byzantine empire means that you have to deal with an unbroken line to Rome and also Christianity