When you downvote somebody else's answer, they lose 2 rep and you lose 1 rep. From the FAQ:
You lose reputation when:
- one of your questions or answers is voted down/not useful: −2
- a post where you had successfully suggested an edit has been deleted (reputation page shows the cause as removed): −2
- the account of a user who was the final approver of a suggested edit you made has been
deleted (reputation page
shows the cause as User was removed): −2
- you vote an answer down/not useful: −1
This is done to balance prevention of malicious downvoting on (possibly competing) answers vs encouragement of downvoting useless/harmful posts.
As the screenshot shows, you have downvoted an answer there, so you certainly have interacted with that post.
Reputation gains or losses are usually reversed when the post is deleted.
Your downvoting activity is not visible to others (even moderators) to protect you from revenge.