Oxford and Cambridge was founded AFTER the dark ages. The dark ages (5th century to 10th century) is called so, because of the relative lack of written sources, between the fall of the western roman empire, and the establishment of church institutions (like monestaries, Oxford, Cambridge and so on) that generated and preseved text about the time.

I was under the impression that historians no longer use the term 'Dark Ages,' rather using 'Early Medieval' as the definition.

“Dark Ages” is no longer used in academia.