The concept of a “Dark Age” as a historiographical periodization originated in the 1330s with the Italian scholar Petrarch, who regarded the post-Roman centuries as “dark” compared to the “light” of classical antiquity.[1][2] The term employs traditional light-versus-darkness imagery to contrast the era’s supposed darkness (ignorance and error) with earlier and later periods of light (knowledge and understanding).[1] The phrase Dark Age(s) itself derives from the Latin saeculum obscurum, originally applied by Caesar Baronius in 1602 when he referred to a tumultuous period in the 10th and 11th centuries.[3][4] The concept thus came to characterize the entire Middle Ages as a time of intellectual darkness in Europe between the fall of Rome and the Renaissance, and became especially popular during the 18th-century Age of Enlightenment.[1] Others, however, have used the term to denote the relative scarcity of records regarding at least the early part of the Middle Ages.

Source. I use it in the former sense, which I think is more common.