The dark ages is not a synonym for the medieval era. It's a sub-division of the medieval era - one of three.
The dark ages is a period after the fall of Rome. It's named for the fact that very little written output was produced during this time. It ran about 500-1000AD.
The high middle ages came after, and there was a lot more writing going on during this period. This was about 1000-1250AD. When people think 'medieval', this is usually what they think, if not the late medieval period (the third sub-division).
Then came the Renaissance, then the modern period.
The dark ages actually were pretty dark, mysterious, and primitive, as evidenced by the very low intellectual and cultural output during these years, low populations, low levels of social organization, and so on. Generally, the good parts and achievements of the medieval period that you're gesturing at were during the high and late parts.