When you downvote a post, it's considered polite to leave a comment explaining why, but it's not required.
We often see comments from the original poster asking why a post was downvoted. (I've done it myself.) Often the response to such a comment is either silence, or "I didn't downvote, but ...".
How often do downvoters actually come back and explain their downvotes? I'm asking only about downvoters who don't initially leave comments. (If the answer is "never", it implies that comments asking for explanations of downvotes are useless, or nearly so.)
It may not be possible to answer this definitively.
(Part of my motivation for asking this question, assuming the answer is "seldom or never", was to have a place to point when responding to "Why the downvote?" comments. It took me a while to learn that complaining about downvotes isn't particularly useful.)