Historians don’t call the Dark Ages that anymore; that term’s as outdated as the Edsel.
The disparaging label came about sometime during the Renaissance years (from the 1300s to the 1600s) when the common man could read and write, and science and technology were flourishing.
Much of the scientific and technological legacies left by the ancient Romans had been lost in western Europe during the Middle Ages as political instability created a large educational and financial gap between the wealthiest lord and the poorest peasant.
When Renaissance scholars compared the early Middle Ages—a time period from about 400 A.D. to 900 A.D.—with their own lives, the early time period seemed quite dark indeed.
Fortunately, this happened only in one small part of the world, western Europe.
Various cultures—the Arab and Byzantine civilizations, for example—grew and advanced during this same time period and spread their knowledge all across the globe, eventually dragging even western Europe into the Renaissance as well.