is it possible to find out who downvotes a question/answer?
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It is not. The rationale behind it is that this would prompt vendettas/mutual-downvoting-wars etc...
Why do you wish to know?
If it's to ask "Why did you down-vote", your best bet is to add a comment to that effect.
If you believe the down-vote was malicious (especially in a pattern), wait 2-3 days for the system to fix it (So has jobs which nullify serial down-votes) and if not, ask SO team (the email is available on the Contact link IIRC) to look into this. You can also try to attract moderator's attention (either on SO, or by posting details on Meta)
If you just want to revenge the down-vote with one of your own, i somewhat sympathize on a personal level BUT, leaving aside large scale issue like "good of the SO ecosystem", do you think you'd like it if YOUR down-votes would be retaliated against?