The site has an Unanswered Questions list, and right now there are 94 questions on it. That's out of 7327 questions total†, which puts us at 98.7% answered.
Questions appear on this list if they have no answers scored 1 or higher. That means we can do three things to make this list smaller:
If a question is answered, but all the answers are scored 0 or lower, vote! If the answers are bad, downvote; if the answers are good, upvote. If the score on any of the answers gets up to 1, it'll fall off the list.
If a question has no answers, post an answer! If a question is answered in the comments section, you can always post that comment as an answer; just mark it Community Wiki and make sure you give credit to the original commenter.
If a question is unanswerable, unclear, or off-topic and slipped through the cracks, vote to close! That will remove it from the list.
Note that it doesn't matter whether the OP has accepted an answer. If it has an upvoted answer, it'll still be considered "answered" as far as this list is concerned.
Please don't upvote bad answers to remove a question from the list. In fact, if you see a bad answer upvoted to 1, you may want to downvote to get it onto the list. Why? Because unanswered questions get more attention. Community periodically bumps them to the top of the front page, and people can see a list of unanswered questions at any time.
With that in mind, if anyone would like to help make the list smaller, it would make the site a better place :-)
†Excluding closed and deleted questions.