What is one reason that the middle ages are sometimes referred to as the "dark ages"

The Middle Ages are referred to as the "Dark ages" for several reasons:
-It all begin with the fall of the Greek and Roman empires.
-Lack of knowledge in the Scientific subjects, and intellectual difference between the people.
-The fast spread of Fatal maladies that killed millions in Europe.
-A big number of battles and warfare, which caused massive destructions and the death of millions.

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