They were called the Dark Ages because there were so many knights! xD Jk
Between the fall of the Roman Empire and the Renaissance there was a period of cultural and intellectual stagnation in Europe. There was no progress made, no development. in fact things went backwards. This period was known as the Dark age. People consider this the dark age because not a lot of major things happened. A few things here and there but nothing significant. 'The power of the church' would probably the key phrase I would recommend you use, though, because the church was the most powerful influence in the intellectual stagnation of that period. Hope I helped. :)
