When "the galaxy" and "the universe" are used interchangeably, either in the logic of the plot or explicitly.
Since Tropes Are Flexible, it extends to particular planetary systems and even the Earth. In a similar vein, this trope also applies if galaxies other than the main one are treated as if they were a separate universe. In other words, made to be a Planet of Hats instead of a Multicultural Alien Planet like the main galaxy.
Sub-Trope of Sci-Fi Writers Have No Sense of Scale. Related to Everything in Space Is a Galaxy, when the term "galaxy" is misused. Contrast with Small Universe After All.
Compare with The Milky Way Is the Only Way when, despite the narrative acknowledging an outer universe, it's not traveled to; Corralled Cosmos, when there's an In-Universe reason why the characters can't travel outside a given parcel of the cosmos; and In-System FTL, when faster-than-light travel is only used within a star system.
An extreme (sometimes in-universe) form of Creator Provincialism. Related to Land of One City (a city is also the country and vice versa.).
Examples:
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Comic Books
- Marvel Universe:
- The Milky Way is by far the primary setting, to the point that Alternate Universes are born from variations of the Milky Way (Earth, specifically). There have been multiple allusions to other galaxies but it's more to add hype to the big three galactic empires (Shi'ar, Kree, and Skrull) than anything else.
- Galactus is almost always found looking for food in the Milky Way Galaxy, despite the billions of other edible planets elsewhere.
- Guardians of the Galaxy
- The Guardians of the Galaxy do this just by name alone, both original and modern since their remit is protecting the entire universe (whenever they can). This is because "Guardians of the Universe" was already taken by Green Lantern.
- Gamora is often called the Deadliest Woman in the Galaxy. Nevermind there exist galaxies besides the Milky Way with maybe their own deadliest women.
- Star Wars (Marvel 1977): #38: "Riders in the Void": Luke and Leia are thrown clear out of the galaxy when their hyperdrive's control are damaged mid-jump and run across a strange biomechanical vessel floating alone through the intergalactic void, whose pilot-computer gestalt describes itself as the sole survivor of a distant galaxy stripped clean of complex life by a devastating interstellar war.
Films — Animation
- Steven Universe: The Movie: In the series, the Gems' Empire is mentioned as having colonies in multiple galaxies. However, The Movie uses "galaxy" and "universe" pretty interchangeably. For instance, the prologue states that the Diamonds "conquered many worlds across the galaxy," but a signal to all their colonies is referred to as a "message to the universe."
Films — Live-Action
- Star Trek V: The Final Frontier: The Great Barrier was planned to be located at the center of the universe but is changed to be placed at the Milky Way Galaxy's center instead. Beyond the Great Barrier is Sha Ka Ree, where creation is said to have begun as if the center of the Milky Way Galaxy were the center of the universe.
Literature
- Animorphs: Ellimist and Crayak are embedded into the fabric of spacetime and know that they could destroy the universe. But they only play their Great Game in the Milky Way. Ellimist alludes that there exist beings like himself and Crayak in other galaxies — Crayak himself originally fled to this galaxy to escape one!
- Isaac Asimov's Foundation series:
- Second Foundation: The stories originally handle the difference between the Milky Way galaxy and the universe (including multiple galaxies) correctly, but Taglines and back cover blurbs from Panther and Avon conflate the two as if they were synonymous.
- Foundation and Earth: Despite the Milky Way being the only galaxy in terms of plot throughout the whole Foundation saga, it's discussed that humankind has the narcissistic tendency to believe its home galaxy is the only important bit of the universe.
Golan Trevize: Yet we speak so much and so often of the Galaxy that it is all but impossible for us to see that this is not enough. The Galaxy is not the universe. There are other galaxies.
- Halo: Silentium: The Precursors claim to be hundreds of billions of years old, far older than the universe. Why they remained in the same galaxy to be wiped by the Forerunners is anyone's guess.
- The Hitchhiker's Guide to the Galaxy: The Milky Way is the only galaxy ever specifically mentioned, even though the outlandish technology we see would presumably make intergalactic travel feasible — the Improbability Drive technology, in particular. The most notable references to "the Universe" are either metaphysical ("the Answer to Life, the Universe, and Everything") or temporal (the titular "Restaurant at the End of the Universe" is permanently located at the end of time itself). There is at least one mention of a wider physical universe when someone speculates that the enigmatic Galactic President may in fact rule all of it.
- Star Wars Legends:
- The setting primarily sticks to the explored portions of the primary galaxy, with the galaxy's borderlands themselves —the fringe of stars around its edges, alongside rogue stars and dwarf galaxies in orbit around it— being the dark, unexplored frontier. Mentions of truly external galaxies is rare. Specifically, while there are other galaxies, there's a hyperspace disturbance at the edge of the main galaxy preventing travel in and out with only one known place where the barrier doesn't work that leads to a small number of systems outside the galaxy like Kamino and the satellite galaxy known as the Rishi Maze that is mostly unexplored. So, pragmatically, the galaxy is the universe in Star Wars.
- Outbound Flight: The Jedi Master Jorus C'Baoth commissions an extra-galactic expedition, the Outbound Flight Project, in the last years of the Galactic Republic to seek out new life and civilizations beyond the galactic disk. Unfortunately, it ends disastrously thanks to Chancellor Palpatine who sees it as a nice way to get rid of some Jedi ahead of schedule.
- New Jedi Order: The Yuuzhan Vong are an alien race who hail from another galaxy and invaded the Star Wars galaxy after traveling the intergalactic void looking for a new home for their civilization after they destroyed all life in their home galaxy.
Live-Action TV
- Doctor Who: The Time Lords oversee the universe in a Custodianship fashion, yet all of their interventions involve the Earth and the Milky Way in some way or another. The titular character is particularly faulty of this thanks to his fascination with human beings.
Tabletop Games
- Isaac Asimov's Robots: The cover of the game conflates the fate of Earth (which you can save by correctly solving the mystery) with the fate of the galaxy (which the cover claims it's at stake).
- Warhammer 40,000:
- The Eldritch Abomination Gods of the Warp live in the "immaterium," a sort of Spiritual counterpart to the Milky Way, and want to merge the two. Whether this extends to the rest of the Universe is up for debate. One argument in favor of this trope is that the said chaos Gods are the embodiments of rage, lust, hope, and love felt by the sentient species. Since they hail only from sentients in the Milky Way, it's as if there's not much out there.
- The Tyranids are extragalactic insectoids that differentiate themselves from the main galaxy's sapient species in that they are the ultimate predators and are all variations of the same genetic theme. Essentially, an Alien Invasion of galactic scale.
- For humans, FTL Travel is only possible in the range of the psychic beacon of the Astronomican, which is on Earth and doesn't even cover the totality of the galaxy (and, as of the 8th edition, has been cut off from about half of its former range). So for all intents and purposes, there is no universe outside the Milky Way.
- Warhammer 40,000: Inquisitor — Martyr: Carvings decrypted from their ruins about "the grand pilgrimage" and "the world of strange stars" leads a tech-priest to a far-fetched theory that the Fabricatus race is not extinct, but instead left for another galaxy for an unknown reason. Said tech-priest also speculates if the Fabricatus were considering the possibility of an impending apocalypse.
Video Games
- Halo 4: In the marketing campaign, the Big Bad Ur-Didact is announced as a threat to the entire universe when, in fact, he's "only" a threat to the Milky Way galaxy.
Set in the aftermath of Halo 3, Master Chief returns to confront his own destiny and face an ancient evil that threatens the fate of the entire universe.
- Mass Effect: The Reapers are sentient machine spaceships that reside just outside the rim of our galaxy and return every 50,000 years to forcefully convert sentient species into more Reapers. Sentient species that have achieved space travel, that is, so they can purge them and try to prevent them from entering the galactic, Vicious Cycle of evolution peak/extinction. Guess only the Milky Way hosts such species since the Reapers spent those 50,000 years dormant, not, you know, traveling to other galaxies.
- No Man's Sky: Double Subverted. The main storyline guides the player across the vast Euclid galaxy until the Galaxy Centre is reached, which allows intergalactic traveling. Nevertheless, it's discovered that the universe is in need of resetting and that means replacing the Euclid with another kind of galaxy. Therefore, the countless other galaxies are just parallel iterations of the original Euclid. After all, they are all simulations generated by the same AI, the Atlas.
- Super Mario Galaxy: Exaggerated. Rosalina claims that Earth's sun is the center of the whole universe. So, it's less "what other galaxies?" and more "what other planetary systems?." This is coupled with Everything in Space Is a Galaxy given that the individual levels are called galaxies yet, in truth, are no more than weirdly-shaped planetoids. Oh, and groups of five galaxies comprise a dome. In turn, the Galaxy mentioned in the title is the complete collection of domes. Yeah, confusing but fun.
Western Animation
- WinxVerse: The Great Dragon is stated to have created the whole universe, spicing it up with life and magic. Now, the universe is called Magic Dimension (or Magic Universe or Kingdom, depending on the dub), but the said dimension is composed of a number of planets, realms, embedded sub-dimensions, and unnamed stars that all rotate around the planet Magix. So, the Magic Dimension as a whole is more of a galaxy than an actual universe. The only times where space maps have been shown have been in seasons 3 and 8 and the first website — all three instances show the Magic Dimension arranged like a big, physics-defying planetary system (not helped by the fact it's hard to pin down the exact difference between a planet, a dimension, and a realm). Funnily enough, the Earth's situation is very ambiguous, to say the least. It's outside the Magic Dimension, but it's unclear whether that means the Milky Way and the Magic Dimension are fellow galaxies or fellow dimensions/universes. The fact the Earth used to harbor magical beings of unclear origin (are they conquerors or natives?) further muddies the waters since it implies that it could belong to the Magic Dimension.
Real Life
- This was the view held by scientists until The Roaring '20s, when it was clear that a lot of objects considered as "nebulae" were actually galaxies (more or less) like our Milky Way. What finally tipped the balance was the realization and subsequent debates that what we now know to be the Andromeda Galaxy had to be farther than the universe's estimated size at the time. For a while afterwards, galaxies were often referred to as "island universes" as a lexical holdover of this model.
- In the very distant future (2 trillion years from now, over 100 times the current age of the universe), if the Universe keeps increasing the acceleration of its expansion, even the gamma-rays emitted from galaxies beyond the Local Group, which will have merged into one super galaxy by then, will have a wavelength longer than the size of the observable universe, putting them beyond the cosmological horizon and making them undetectable in any way. At that point, an observer inside their own galaxy would look out and see... nothing at all. Any conclusion that they live in a larger universe with many galaxies will have to be done either by inference, or by reference to truly ancient records, rather than observation. Some astronomers have speculated that in such distant times, cosmology could become like a religious belief. The only way for beings in those times to know how the Universe was born, and what's out in space, would be records from their past. Belief in those records would vary with how much those records were trusted, and the evidence in them would no longer be verifiable.
