The idea that knowledge didn't disappear during the "dark ages" is blatantly wrong. Entire swaths of knowledge disappeared simply because the Christian monks only preserved matter favorable to Christianity.

If the Roman (pagan) material dared to suggest something different than accepted church teaching, it wasn't copied or preserved.

Hell, that's why the capture of libraries in Spain in the 15th century was still important to the renaissance. Muslim scholars preserved a bunch of Greek and Roman material that had COMPLETELY disappeared during the European dark ages.