I've seen a perfectly reasonable answer, others' as well as my own, where I see a downvote, but no comment to substantiate it.
Just thinking out loud, would it make sense that a downvote attempt causes a comment dialog box to appear and the comment is required to enter the vote. On a different stackexchange board, I don't recall the exact topic, but walked away thinking "who downvotes cheesecake?"
On this board, obviously, a factually incorrect answer should get a downvote, but I frequently see it for nothing I can find.
EDIT - OK. How about instead of a comment, there are choices as with a close vote. Say (a) factually incorrect, (b) doesn't answer question as asked, (c) (something else?)