Fan fiction lets people get closer to their heroes. It lets them get close to celebrities, step into their worlds, imagine their lives intertwining with their idols'. And it also lets them have sex. Lots and lots of sex. The rise of fan fiction — specifically of the erotic variety — has prompted fan fiction megasite Wattpad to release a free iOS app, called After Dark, to collate its thousands of romance stories.

The app is curated, breaking its mature-rated stories up by content-specific channels such as "southern romance," "urban," and "panty droppers." As on Wattpad itself, users can comment and vote through the app, which for now, is only available on the App Store. The app will also feature regular romance stories alongside its fan fiction, but Melissa Shapiro, Wattpad's head of marketing, says it's the celeb-focused stories that drove the app's creation. "On the heels of the release of Fifty Shades of Grey, interest in mature stories has spiked," Shapiro said. The BDSM-centric story was first conceived as a Twilight fan-fic by "Snowqueen's Icedragon," a pen name for writer E. L. James. She said the story was her "midlife crisis, writ large," and featured "all her fantasies" — an attempt to inject the fairly chaste vampiric love story with more raunchy sex.

Fan fiction is not a new phenomenon. Established writers have set their stories in the worlds of Sherlock Holmes, Cthulhu, and Conan the Barbarian, to name but a few, using characters invented by original authors to tell new stories. But the internet helped the concept grow and develop — now it's got its own genres, tropes, and arcane terminology. It's also growing rapidly in popularity: Wattpad now has more than 40 million monthly users and hosts more than 100 million uploaded stories.

After Dark may also end up hosting the next Fifty Shades of Grey. Publishers and movie studios are increasingly using fan fiction sites such as Wattpad to find new material, with fan-fic author Anna Todd, who received a six-figure publishing deal for her series After, the most recent success story. Todd's story focuses on 18-year-old Tessa, who meets One Direction member Harry Styles and falls in love with him, before having several bouts of carefully described intercourse. Paramount bought the film rights to the series in October.

The deal shows fan fiction at something of a transitional stage, popular enough to warrant huge book and movie deals, but still seen as something to hide by some in its community — the 25-year-old Todd hadn't told her husband about her hobby or the huge online following she'd built up, only revealing her secret after being offered a book deal. The release of After Dark is one of fan-fic's largest publishing platforms showing tacit approval for the form's steamier side, erotic offerings that don't define all of fan fiction, but nonetheless — as Fifty Shades of Grey shows — can become incredibly popular.