In a lot of Space Opera there is a single political entity that governs the majority of the setting, even if the setting spans an entire galaxy, or universe in some cases. It may be an Empire, a Republic, a Federation, some even work more like a Fictional United Nations, it doesn't really matter. If there are other polities they are small nations at the outskirts of the Empire or rebellions against them.
There might be several reasons why a writer would employ this trope:
The writer doesn't want to go to the extra work of creating and developing lots and lots of tiny nations and creates a dominant Galactic Superpower to save time. And to be honest detailing more than one nation is not really necessary unless the story involves politics.
The writer is writing a story of the overthrow of a great galactic tyrant.
The superpower fell sometime ago, and the story may be about the attempt to restore it.
The Empire of Valdana in Tytania ruled space at the beginning of the series. But apparently the ruling Tytania clan wasn't as powerful as they thought.
In Legend of Galactic Heroes the Galactic Empire rules over the majority of the galaxy, with only two other factions even existing. The neutral Phezzan Dominion, and the Free Planets Alliance, which is in fact an offshot of the Galactic Empire and significantly smaller, although it can be said to be a Galactic Superpower in its own right. Reinhard von Lohengramm, as emperor of the Galactic Empire, has the unification of the entire galaxy as his ultimate goal. He more or less succeeds.
Star Wars had first the Old Republic after the defeat of the Sith Empire, then the Galactic Empire, then the New Republic, then the Galactic Alliance coexisted with the Fel Empire for a century before the Sith took over the Empire again and conquered most of the Alliance.
Predating the Old Republic was the Rakatan Infinite Empire, a race of Abusive Precursors that enslaved most of the galaxy, then were done in when the Force rejected them.
The generally unnamed Empire in Dune that formed after the Butlerian Jihad and lasted up until Leto II manipulated its breaking up (almost 14,000 years).
The CoDominium universe went through three. The first was the titular alliance between the U.S.A. and U.S.S.R., the second was the Empire of Man founded by the former colony world of Sparta after the founders nuked each other into oblivion. The third was a second Spartan Empire that succeeded the first.
Honor Harrington is unique in that it initially took place in the "fringe" rather than anywhere near the superpower. The Solarian League is so vast that most maps don't show the entire thing, but their sheer numbers have meant that no one dared to fight them for centuries, as such the "barbarians" like Manticore tore their fleets to pieces when the League was manipulated into trying to extend their reach that far.
In the Foundation series there's the Galactic Empire, which is in the process of falling at the beginning of the first Foundation story, and later the Foundation itself, which is designed by Hari Seldon to be the new Galactic Empire after "only" 1,000 years (instead of the 30,000 years he anticipates chaos will reign without the Foundation in place).
Averted in the human era of Larry Niven's Known Space universe but two billion years ago the Thrintun (or Slavers) conquered most of the galaxy using their telepathic abilities to enslave other species. Until one of their slave races, the Tnuctipun, rebelled en masse and instead of admitting defeat the Slavers commanded every chordate in the galaxy to commit suicide.
In the Star Carrier series the human government, the Terran Confederation of States, is one of the small fringe factions. The series deals with their efforts to avoid being subsumed or destroyed by the Sh'daar Masters, an empire that controls most of the galaxy through a variety of vassal species.
In the Distant Finale of Star Trek: Federation set many centuries in the future the UFP has unified the entire galaxy, and ships are now fast enough (using something called a "sidewarp" drive) that crossing it is trivial.
Poul Anderson's Technic History series had its Terran Empire phase, in the 31st century, though frequently the stories mention the vast difficulties of it and the limits of the empire's ability to actually control.
In Andre Norton's The Zero Stone, Jern contemplates at one point that the Zacathans found evidence of several galaxy-wide civilizations that predated them -- and the Zacathans themselves have millions of years of history recorded.
Stargate Verse: For a good twenty millennia prior to Earth's stargate program, the Goa'uld Empire ruled most of the Milky Way with only a few small pockets resisting them. The fact that the Tau'ri were able to do in a decade what the Asgard, Furlings[[note]]according to the RPG they were involved in some of the fighting against Ra around the time he discovered humanity[[/note]] and Tok'ra couldn't do in thousands of years is quite impressive.
The Federation in Blakes Seven initially controlled all of the settled galaxy, except for some outlying settlements, the anomalous Aurons, and a few insular and well-hidden aliens.
The Anglo-Sino Alliance has jurisdiction over almost the entirety of the star system in which the Firefly franchise takes place (a half dozen stars with attendant planets and moons), though this is only very recent. Prior to the Unification War six years ago only the inner planets were members; the outer ones were independent. How much control they actually have varies from planet to planet.
The Hitchhikers Guide To The Galaxy has a galactic empire that Zaphod Beeblebrox used to be president of. And which seems to have nothing better to do than demolish planets to build hyperspace express lanes or chase after the ex-president.
There was also a hugely prosperous empire that collapsed five million years ago, due in large part to everyone's money going to Magrathea's luxury planet industry.
The Imperium of Man in Warhammer40000 spans the Milky Way galaxy, but there are many worlds within that expanse that are controlled by other factions which the Imperium is constantly at war with. And the Eastern Reach where the Tau reside is still being explored.
The Terran Hegemony and Star League in the backstory of BattleTech ruled over most of known space but ever since it broke up the four successor states and the Clans have been pretty much in a stalemate.
The three Imperiums in Traveller were largely this in their times, though the Third Imperium encountered similarly sized alien Empires on its borders.
The Terran Confederacy in StarCraft was thought to be the sole power in the Koprulu sector until contact with the Protoss. After its fall the succeeding Terran Dominion is the largest Terran polity in the sector but has competition from the Kel-Morian Combine and the Umojan Protectorate.
Subverted in Mass Effect. Most of the known Milky Way is controlled by the Citadel Council, but because of the limitations of mass effect drive and laws against opening mass relays willy-nilly barely 1% of the galaxy is actually explored.
50,000 years ago the Prothean Empire dominated. And in previous cycles other empires ruled, the Citadel Council is implied to be an unusual method.
In EV Nova the closest thing to this is the Auroran Empire, which isn't a single polity so much as a loose confederation of warrior clans that fight among themselves as often as they fight the Federation or the Polaris. Collectively they control a huge swathe of territory along the bottom half of the map bigger than the other two superpowers put together.
In the Starfleet Adventures mod for Nova (a total conversion for 23rd-century Star Trek) the Federation controls a patchwork of systems that amount to roughly half the map (which covers both the Alpha and Beta Quadrants), with other polities (Klingons, Romulans, Cardassians, Gorn, etc.) interspersed among them.
The Pangalactic Federation in the Star Ocean series.
Homeworld: The Taiidan Empire. In ancient history the Hiigarans were the dominant power before the Taiidan overthrew them.
In Orion's Arm the First Federation attempted this in the 10th century a.t., more or less succeeding in unifying the Solar System and the few interstellar colonies that existed at the time. But as expansion continued the Federation became more or less a "rubber stamp" body and after about 1800 a.t. it was essentially irrelevant. The Second Federation of 3800 to 4450 a.t. was not so much a polity as a universal protocol of interactions between polities, which pretty much ended with the Version War. In the 106th century a.t. Terragen space is divided between several Sephirotic Empires that are starting to fragment.
Five hats means that five tropers think it is ready to publish.
You are saying that you think this draft is ready to be published. That means the description is not ambiguous,
it doesn't duplicate an existing trope, there are at least three examples, and the title makes sense.
Is that what you meant to do?
You are saying this draft has a ready-to-publish hat it does not deserve and you are taking it back.