dark age

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The period in Europe from the fall of Rome in the fifth century AD to the restoration of relative political stability around the year 1000; the early part of the Middle Ages.
• noun The entire Middle Ages, especially when viewed as a troubled period marked by the loss of classical learning. No longer in use by historians.
• noun An era of ignorance, superstition, or social chaos or repression.
• noun The early or crude stage in the history or development of something.
• noun The period in the history of the universe, roughly the second 500 million years according to current cosmological models, when all matter was in the form of neutral hydrogen and helium atoms, but no stars or galaxies had formed.