If I adopted your thought process I'd be accepting that anything which can be conceived of could exist. Technically you're sort of right in the sense that disproving the existence of stuff is more or less impossible but accepting the probable existence of anything and everything is also problematic. It's a huge jump from "95% unknown" to "possibly demons?".
Just because we don't understand everything doesn't mean we make unfounded assumptions about the nature of the unknown. There is no reason to suppose that demons exist in the same way, as an atheist, I see no reason to suppose that God(s) exist. In our culture demons are part and parcel of religious belief.
As a Christian I assume demons form part of your religious worldview therefore you have reason to include them in your list of things which probably exist.
On the balance of probabilities, I can comfortably dismiss the concept of demons, along with dragons, elves, fairies and leprechauns, unless I get evidence to the contrary.
OB