The subject of the dark ages has been skewed quite a bit over the years what hasn't
The dark ages basically refers to the period after the fall of the roman empire. The schools of thought and philosophy dissolved with the unstability of the empire, and the writings and teachings of the classical latin fell apart. The speech of the common people became the standard, the vulgar latin. Subsequently, remaining rulers and scholars thought them sub par from the classical teachings, again primarily just because of the vernacular.

The only place to receive classical teachings became the church. Churches that held the remaining literature and necessary study materials were few and far between. It wasn't the church that darkened and restricted any scholarly achievements or anything, it was just hard for even the oligarchs and aristocrats to engage in study. Top that off with the lack of roman stability there were mass incursions and conquests from the slavs, vikings, saxons etc.

It is called the dark ages because there is very little in the way of historiographic texts and philosophies by which to study it. As a result, dark ages day to day life is rather unknown to us in modern times. A better term would be "little studied".

There are archaeologic dark ages, geologic dark ages etc. It just refers to a time period for which there is little to no evidence or remains which can be studied. It doesn't refer to an oppressive power darkening the intelligence of the world or whatever the fedora wearing MLP fandom believes. Religion was not seen as contrary to rational thought and reason until recently. In fact, the church taught Grammar, logic, and rhetoric, arithmetic, geometry, the theory of music, and astronomy. These were known as the liberal arts(artes liberales) (liberal arts taught in the dark ages, weird) and considered necessary for all free men to properly contribute to civic society, it just wasn't possible in the virtually constant state of warfare for roman europe.

Achievements of the dark ages:
Gothic architecture (yes all of it, engineering of cathedrals hit its peak here and revolutionized architectural design)
traditional motte and bailey castles w/ moat
astrolabe (a century without crusades and the arabs got bored fighting each other so they invented **** )
Monastaries and universities (basically temples devoted to recovering and retaining classical literature and sciences, funded by the church)
The heavy Plow (deep furrows, turn over clay, which was previously untillable except by hand)
Crop rotation (increased yield)
circumference of the earth first estimated (no they didn't believe it was flat, and most people didn't give a **** one way or another)
Trade laws that transcended city and state boundaries