Soldiers of the Dark Ages
The Dark Ages, or Middle Ages, was a period in Human history known for its barbaric medievalism. This was a period of great darkness, a time when Humanity was beginning to "lose their way", being confronted with wars and plagues. It was followed by a period known as the Renaissance. (DIS: "Die Trying")
In 1986, when Doctor Leonard McCoy encountered a hospital patient that was going to receive a kidney dialysis, he was shocked that they were still doing that in the 20th century. He then said "What is this, the Dark Ages?" (Star Trek IV: The Voyage Home)
When the landing party of the USS Enterprise visited the Guardian of Forever, the Guardian showed them many eras of Earth's history, including the Dark Ages. (TOS: "The City on the Edge of Forever")
In 3189, while talking with Fleet Admiral Charles Vance, Captain Saru told him about the Renaissance artist named Giotto, whose paintings brought forth a new era in Human history, thereby ending the Dark Ages. (DIS: "Die Trying")
While Saru described the Dark Ages as a period of history for Humanity as a whole, in reality the concept referred only to the history of Western Europe, and is widely regarded as inaccurate even in that context.