I believe a user is using malicious down voting as a tactic for getting his answer higher than the one posted before him, then when challenged he an a friend down voted to the cap on answers of mine from a while back. I believe this is a very negative behavior on such a positive site. Please help.
If you have been serially downvoted, the downvotes will be automatically reverted by a script.
Users that resort to serial downvoting often enough will be flagged for moderator review and can face temporary suspensions if they don't improve.
You can flag the comment where you challenged the user for the behaviour for moderator attention if you feel the user is consistently downvoting competing posts. A moderator cannot see who downvotes either though, but it may be that with some careful observations the user can be called out on the behaviour anyway.
These downvotes will be removed by an automagic script which detects voting fraud, usually within a day (~12-24h tops). If these have not been removed within a day or so, flag one of your questions or answers which has been downvoted with an Other message for a moderator explaining exactly what you think is going on.
However, just because you believe this is going on does not mean it is. Voting is anonymous and your questions/answers may have garnered downvotes the old fashioned way.