When the trailer for Alex Garland’s thriller “Civil War” was released in December, a vast input-output machine creaked into gear. Fans combed and screenshotted the preview for clues about the movie’s dystopia. YouTubers reacted to the brief, tense scenes. Bloggers attempted to sketch out the politics of its imagined second American civil war. A Reddit user isolated and reconstructed a map shown briefly on a television screen — an apparent Rosetta stone to the deep lore fans could look forward to inhaling.

“Civil War” would follow four journalists traversing a bombed-out Mid-Atlantic. But commenters were less interested in the plot, the actors or even the tone than they were in outlining the history and political economy of a fictional world they had glimpsed for just 2½ minutes: How had the civil war started? What were the sides? Why was California allied with Texas?

If it felt like overkill for a movie with no preexisting fan base or intellectual property, well, this is how we watch now. We live in the era of the “explainer movie” — a creation like those in the Marvel Cinematic Universe, Dune ecosystem, Star Wars constellation and, on the television side, the Game of Thrones series that has sweeping and sophisticated fictional worlds, extensive background lore and complex (or, at least, many) narrative arcs. Such properties are, in other words, productions about which explainers get written, and they have supported a flourishing ecosystem of interpretation and exegesis, occupied by columnists, bloggers, YouTubers, Redditors and X posters at every level of fame and respectability. You can have a random Reddit user called u/Ynot1989 nitpicking the “Civil War” map or New York Times columnist Ross Douthat weighing the political systems of Westeros in “Game of Thrones.”

“Civil War,” a narrowly conceived story about political violence and war journalism, was not meant to be analyzed like this. It had no “cinematic universe,” no context to be synthesized. But can you blame Redditors and YouTubers, fed on a steady diet of elaborately overlapping comic-book movies and extravagant adaptations of complex science-fiction and fantasy epics, for assuming otherwise?

The trailer for “Civil War,” starring Kirsten Dunst, center, was dissected for world-building clues when it was released last year. (A24)

Over the first quarter of the 21st century, the transformation of the entertainment business by the internet and Hollywood’s obsession with preexisting intellectual property have helped turn almost every blockbuster into an explainer movie. An expansive mythos and sophisticated world-building are all but production requirements for these tent-pole releases — the way a happy ending and smoldering romance might once have been — and vast resources are marshaled to ensure that the fictional worlds of a given movie and its sequels, prequels and “sidequels” are internally consistent and intentionally wrought.

It’s maybe getting … to be too much. The second movie in director Zach Snyder’s “Rebel Moon” saga (about which the director has said, “I’ve spent the last two or three years building out this universe. Every corner has to be painted in.”) seemed to debut and disappear on Netflix with almost no attention. “Civil War” made more than $122 million at the box office, a huge sum for its studio A24, which didn’t bother with a tie-in prequel series or even a concrete explanation of the movie’s political conflict. And the Marvel Cinematic Universe has, in recent installments, become as frustrating and continuity-plagued as its crosstown rival, the DC Extended Universe.

But hit refresh. Robert Downey Jr. has reentered the Marvel Cinematic Universe as Doctor Doom, as the entertainment juggernaut seeks to restore its old magic, and DC Comics’ film efforts are undergoing a cape-to-cowl refresh. Throw in a new Dune streaming series this fall, and it’s clear explainerism isn’t going away.

How did we get here? I’ll explain.

Explainerism — the entertainment mode that attempts to satisfy the unquenchable impulse to overthink and pedantically document a fictional setting — long predates the cinematic universe. Sci-fi and other genres’ fans have for, decades, reverse-engineered entire worlds from their favorite stories. Like Sherlock Holmes fans, who since the 1920s have played “the Sherlockian Game” in which they try to puzzle together consistent explanations for inconsistencies in the mysteries by Arthur Conan Doyle. “The Lord of the Rings” author J.R.R. Tolkien famously invented several languages for Middle-earth, and his son Christopher ensured that the exhaustive (in more ways than one) lore was completed and published after Tolkien’s death. Star Trek and Star Wars had such devoted and inventive fan bases that creators Gene Roddenberry and George Lucas had license to deepen and interconnect their fictional worlds between franchise installments — a Klingon language and dictionary was created for Star Trek in 1984, when most sci-fi movies were just having their alien actors speak gibberish.

But, for the most part, science fiction and fantasy of the 20th-century blockbuster era was a mix of specific and limited original stories (like “Legend” or “Stargate” or “The Fifth Element”) or adapted intellectual property (like the four Batman movies made between 1989 and 1997, or even David Lynch’s 1984 “Dune”) that was significantly streamlined to remove extraneous, and presumably onerous, mythology. From a continuity perspective, franchises like the Planet of the Apes or Jaws never got too complicated.

I suspect that many nerds (myself included) look back on all this inventive and groundbreaking sci-fi and fantasy quite fondly now, but I remember finding it quite frustrating at the time. Complex fictional worlds are one of the main attractions of speculative-fiction fandom (alongside “cool spaceships” and “laser guns”), so “streamlining” really means “dumbing down.” The increasingly large (and increasingly rich) population of sci-fi fans were theorizing, speculating and arguing, at conventions and comic-book shops and, increasingly, on bulletin board systems and online message boards, but this intense passion was rarely reflected on-screen.

But as far as Hollywood was concerned, extratextual lore — extensive biographies of background characters or prequel stories not directly told in movies — was the province of toy manufacturers and pulp publishers, to whom studios were happy to delegate the explainer function and associated questions of canonicity. Lucasfilm leased the Star Wars universe in the 1990s to publishers like Bantam and Del Rey. This meant science-fiction writers like Timothy Zahn and Steve Perry, whose novels, some of which were set contemporaneously with the films, filled out explanations for what went unseen on-screen and elaborated an even more complex “expanded universe” than what Lucas had depicted in the original trilogy of films. These novels were devoured by fans but were extraneous to the movies and were regarded with amusement (at best) by Lucas.

That arrangement seems quaint now. In the 21st century, the central dynamic has flipped: Sherlockian levels of close reading and Star Trek levels of pedantry have become the norm for nearly all fictional universes. The Game of Thrones and Dune franchises not only have (several) constructed languages for the actors to use, but they also have fully realized alphabets for those languages that are used “correctly” and consistently even in glancing background shots, a level of rigor that would have once been unthinkable. Blockbuster sci-fi movies are assumed to be building blocks in an expanded universe with lore under centralized studio command. And the new minder of Star Wars — Disney — wiped the earlier novels from the official canon. Only works under its supervision “count” — new novels, comic books, Disney Plus series and future films. No moment in the Star Wars universe can be left un-depicted.

Meanwhile, Hollywood counts on audiences to keep up with that lore either by consuming it directly or through all those third-party explainers. If you tuned into 2022’s “Doctor Strange in the Multiverse of Madness,” you might have been surprised that the heroine Scarlet Witch had become a villain, and the film offered very little explanation. But that important background information came in the 2021 Disney Plus streaming show “WandaVision,” and woe to anyone who didn’t watch it!

What changed? If one work could be said to have unleashed explainerism on Hollywood, it’s “Lost” — the surprise-hit 2004 ABC series about a group of strangers stranded by a plane crash on a mysterious island. Although “Lost” never spawned interlinked spin-offs, its success contravened Hollywood conventional wisdom about the dangers of intricate world-building and overdependence on continuity. It turned out that not only were large audiences willing to tolerate TV shows that required a great deal of explanation, they were all too happy to do that explaining themselves on message boards, forums and nascent websites that would soon be known as social media platforms.

While “Lost” may have been the proof of concept, it was these online communities — arising out of existing sci-fi and fantasy fandoms, and those of many others — that enabled the show’s success. The explainer movie is a natural response to the changes wrought upon Hollywood by the internet and all it entails — the changes to fan culture, to entertainment journalism, to distribution and even to the composition of the audience.

On a practical level, the web has changed the basic conditions of reception for any fictional universe. Anyone curious about a hit movie or TV show’s particulars can access every theory and explanation you can imagine. Wondering about the midcredits scenes in “Deadpool & Wolverine”? Don’t worry, many outlets can describe and interpret them. Questions that would once have been the province of fanzines are now provided by the studio — HBO’s website features a Dragon Index for curious viewers of “House of the Dragon,” a “Game of Thrones” prequel. And the endless, hyperlinked structure of the internet all but demands a fictional world with deep rabbit holes. The fan-created wiki site Fandom boasts “40 million content pages in over 80 languages on 250,000 wikis about every fictional universe ever created.”

Even more powerfully, the platforms where these explanations live encourage readers to engage and respond, fueling an explainer flywheel of debate and clarification. A world where you can easily screenshot stills from a movie drives this impulse: Every prop, every background, every costume can be dissected and analyzed for clues. Wondering about the text on the message sent by Paul Atreides to the emperor in Denis Villeneuve’s “Dune: Part Two,” which appears on-screen for about two seconds? Turns out it’s a real message, which a number of Redditors have deciphered.

All these moviegoers craving explanation have motivated digital media outlets to create them. The “explainer” component of explainer movies is, at least in part, a product of the metrics culture and incentive structure that distinguishes online media from its print predecessors. Editors and creators can see popular searches and discussions about a property and immediately produce an explainer, which they might presume will attract a sizable audience. Depending on how dignified they’re feeling, they can even scrape “fan theories” from Reddit and repackage them for new eyeballs.

Either way, blockbuster explainers have been very good business for everyone, be they magazines or YouTubers. They’ve been even better business for movie studios, who can count on explainers (in whatever form) as “earned media,” so long as they preload their movies with fan service, Easter eggs and lore to be unpacked.

The result is that big-budget sci-fi and fantasy productions are launched not just with high-profile advertising campaigns and marketing tie-ins, but also with homework assignments: To fully understand what you’re about to see (or what you’ve just seen), you need to complete the reading.

All of this is entwined with the increasingly important, and well-understood, role of intellectual property to Hollywood, which hopes that preexisting knowledge of characters and stories — in sci-fi paperbacks like “Dune” or non-narrative toys like Barbie — acts as baked-in marketing. But explainer movies stand out even among other IP plays, because what’s important is not simply high levels of consumer recognition but the depth and sophistication of the world being depicted.

Which suggests one other driving factor (and this might even be good): an increasing openness on the part of audiences to engage with structurally and politically complex worlds. Perhaps we want to see our escapist media match the level of intricacy that we understand exists in the real world.

Or maybe it’s easier to spend hours reading Reddit threads than it is to spend hours reading the news.

What does the future look like for explainerism? Certainly the environment in which explainer media has thrived is changing: Search engines appear to be becoming a less-reliable generator of traffic for publications, which may be less eager to implicitly partner with Hollywood if their explainers no longer get clicks. Streamers are cutting costs and shows.

And even amid Marvel’s attempt to regroup, there are signs studios are imagining somewhat less info-dense blockbusters. “Alien: Romulus” refers in some ways to the complicated lore of predecessors like “Prometheus,” but it’s largely a stand-alone thriller in the mode of the first “Alien” movie. Like “Civil War,” “Romulus” has been a box office and critical success — suggesting that filmgoers are okay forgoing any franchise homework.

Sci-fi fans who once complained about the simplicity and laziness of world-building in Hollywood blockbusters should be basking in the riches of an entertainment industry producing fastidiously conceived universes. But even we know it’s become too much of a good thing: an overabundance of what can be explained, to the detriment of the stuff — a mind-warping image, a provocative question, the thrills and chills of a shameless genre exercise — that simply can’t.