We all have received a downvote on our questions, and for most of them we wondered why. Does Stack Overflow do anything to prevent downvotes made just for fun or wasting their time?
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For the downvotes on your questions which come in huge quantities from a single downvoter (that is, serial downvoting), there are filters in place which will detect and reverse them. This is afaik a job which runs once each day.
As for the occasional unexplained downvote, no, there is nothing done against those. Nor is there anything that can be done against those. How would you detect that the downvote is invalid? Now, granted, there have been various proposals for mandatory comments on downvotes and downvote validation, but these have all been quite heavily shot down for imho very good reasons. Just have a search through Meta to read through some such proposals. Comments are however certainly encouraged. The basic principle here is that downvoting is a privilege. Once you earn the privilege, you are free to cast your downvotes for any reason, or no reason at all, as long as you're not cheating.
And if you do receive an unexplained downvote, don't just ignore it. Have a good look at your question or answer. Is there anything that might be unclear? Something you can improve upon? A mistake that has gotten into your post? You never know what you might have missed. But if nothing stands out, just shrug it off. Downvotes happen.
There are a number of ways the system helps mitigate this.
While of course none of these things make the problem go away entirely, they do help to significantly limit how often it occurs and the negative effects when it does occur. What's left after the effects of these factors is something that you can live with, and it's something that most users see at least a little bit of. Fortunately I've never seen it really be a problem. It happens infrequently enough, and the effects are small enough, that you really can just ignore it.
I would propose a function where you can add a comment to a downvote where you describe why you did so.