Why are the Middle Ages sometimes called the Dark Ages?

I assume the middle ages are called the dark ages because that is when the great depression was or something im not sure

united states reinforcements contributed greatly to three allied victories in france at: cantigny (first major american battle), argonne (the largest in the united states military history, involving 1.2 million american soldiers), and château-thierry (one of the first actions under general pershing).

i think its 5! hope i