Normal interpretation of "akuma to yobareru dai roku kakuu youso" (literally, the 6th imaginary substance called demons) is that demons, (and not the English "demonic") are the 6th imaginary substance themselves. Materialization, or better yet, physical manifestation of the imaginary substance "demons" requires images made by man.

Other interpretations of the above line are that demons are a particular category within the 6th imaginary substance, but contextually, it seems to be out of place in the segment of Fate where it was mentioned. Well, it would be easier that way though.

Reality Marbles are/and were originally the alien common sense of demons. Real demons don't have a physical body and can only come into physical existence after obtaining an image crafted by us humans. Furthermore, the bodies that demons, when possessing a host, try to recreate are ones that are physically impossible for any organic life. Finally, DDD gives an impression that demons are intangible and incompetent, the complete opposite of a concrete and all-present being such as God and some comments in Character Material makes it seem as if Merem's demons themselves are a contradiction to reality.

Maybe the 6th substance is really the summed up named of undefined and utterly alien nonsensical spiritual energy (if it's supposed to be significantly different from the 5th substance, Ether)? Maybe something similar to the aliens in the Melancholy of Suzumiya Haruhi, but opposite in another way.