IF you are neither 15 years old nor the sort of person for whom the term fan fiction has an ounce of resonance, then chances are that ''Supernatural'' is not in your DVR queue or even in your frame of reference. The series, now in its sixth season on the CW network, remains an invisibly pervasive cultural phenomenon: enormously popular and yet unknown even to many of those of us who take pride in dabbling in the literature and television of young adults. ''Supernatural'' is one of CW's most watched series, this year attracting an average of 700,000 more viewers each week than ''Gossip Girl,'' even as none of its stars feature regularly in the pages of Us Weekly. To say that the series's young fan base is active is grossly to understate the matter. ''Supernatural'' is second only to ''American Idol'' as the most-talked-about show on the popular Web site Television Without Pity, generating hundreds of thousands of comments, 8 to 10 times the number of posts as those about ''Mad Men'' or ''Glee.'' Like ''Star Trek,'' ''Supernatural'' inspires conventions around the world where even actors who have made minor guest appearances on the series can make a good deal of money showing up and signing autographs. Last year, when TV Guide held a contest that allowed readers to vote for the show they most wanted to see on the cover, they elected ''Supernatural.'' [ABSTRACT FROM PUBLISHER]