For decades, Kirk/Spock has been one of the most popular couples in fan fiction, a genre in which fans write stories using characters from books, movies, TV shows and even real life. (Kirk/Spock spawned the subgenre “slash” — fan fiction about same-sex entanglements.)

Fan fiction is having a moment right now, and not just because “Star Trek Into Darkness” has thrown a luscious new villain, played by Benedict Cumberbatch, into the mix.

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The success of “Fifty Shades of Grey,” originally written as Twilight fan fiction, has major publishing houses intrigued. In February, Simon & Schuster published “Beautiful Bastard,” a reworking of another Twilight fan fiction; the sequel is out May 28. Last fall, Penguin hired a 16-year-old to adapt her One Direction fan fiction into a YA novel.

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Maybe it’s about time. “Fan fiction is one of the great unsung popular literary movements of the past 50 years,” Time magazine book critic and author Lev Grossman wrote this month. His novel “The Magicians” and its sequel riff on the magical worlds of J.K. Rowling, J.R.R. Tolkien and others, so you know that dude reads fan fic.

Fan fiction is like methadone to a heroin addict, offering a come-down from the high of the original creation. It’s a rebound relationship, filling the gaping hole left when a favorite series ends (or starts to suck).

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“It makes you realize that not all of what you love about the characters has to come from the original creators,” says an “X-Files” fan fiction author who writes under the pseudonym Jintian (she did not want her real name published). “It can come from people who love the characters just as much as you.”

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She discovered fan fiction while searching the Internet for information on “The X-Files.” Do that today, and you might stumble on “Happy Generation,” in which Mulder and Scully investigate the disappearance of Santa Claus, or “The 13th Sign,” a story envisioning Mulder and Scully living a quiet life with a baby and a Volvo. (“Scully would never believe it, but all he’d ever wanted to be was normal.”)

Fan fiction goes way deeper than major franchises. The adventurous Googler will find stories set in the ’verses (fan fic lingo for fictional worlds) of My Little Pony, “The Adventures of Tom Sawyer” and Sonic the Hedgehog.

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Then there are crossovers, which merge two or more ’verses. “Pride and Prejudice” in Middle Earth exists, for instance. (Mr. Darcy is a hot elf.) So does a mash-up of “Lord of the Flies” and “Legally Blonde: The Musical.”

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Trekkers have been authoring fan fiction since before the whir of a dial-up modem. They would type their stories up and share them at fan conventions.

“A lot of it was literally handed under the table from one person to another,” says Katherine Larsen, who teaches writing at George Washington University and edits the Journal of Fandom Studies.

Making characters kiss isn’t the only motivation. Many fan fiction writers want to fix what they see as problems in fictional worlds.

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Women watching “Star Trek” in the ’60s may have loved the show but not its depiction of gender roles.

“A lot of the early fan fiction is just giving [women] more power, giving them more say, giving them relationships, and giving them some autonomy within those relationships,” Larsen says.

While the original “Star Trek” series kicked off fan fiction as we know it today, the practice goes much further back: to ancient mythology, where stories of the gods and heroes were constantly reinterpreted; to Shakespeare, who liberally borrowed plots from older works; and to Arthur Conan Doyle’s fans, who wrote their own Sherlock Holmes adventures when Doyle quit.

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“That’s what storytellers have always done,” says Rebecca Tushnet, a Georgetown University law professor and copyright expert who helped start the Organization for Transformative Works, a group that supports fan-created art. “You get ‘The Odyssey’ because hundreds of storytellers have retold the stories.”

And wouldn’t “The Odyssey” be much better if, maybe around book 23, Odysseus locked eyes with Spock?

Where to Find It

Most fan-fic communities have their own sites. A Teaspoon and an Open Mind, for example, is a good starting point for “Doctor Who” fan fic.

Fanfiction.net is a massive, if somewhat impersonal, portal with fan fiction for more fandoms than you could possibly imagine.