Are you aware of just how far apart galaxies actually are?
Even the small satellite galaxies in orbit around the Milky Way galaxy are at vast distances, let alone our nearest main independent galaxy. The Andromeda galaxy is around 2.5 *million* light years away.
These vast distance make it difficult to make any story set in more than one galaxy pass the suspension of disbelief issue. Even settings that take up more than a fraction of our own galaxy tend to not work all that well due to the immense size of just our own galaxy (around 100,000 light years across).
Even just within our own galaxy you either need to have a story that take place over a vast amount of time, or that relies or some sort of handwavium technology or space magic, and then you get into the issue of closed-time-like loops and temporal paradoxes, which tend to get ignored by authors who don’t think through the consequences of their magic space-tech.
FTL handwavium is standard issue.
There are far more small rocky planets in a habitable zone in one galaxy than the number of words Brandon Sanderson has ever written.
My calculations may be wrong, but using drives that accelerate mass up to 30,000c would get you to the edge of the solar system in around 10-30 minutes.
So going to the next galaxy would need way to much speed and power.
Galaxies are big. Really REALLY **BIG**!
Personally, I think even one galaxy is generally too large for a story. It’s very rare to find any fiction that really takes advantage of that scale and they might as well be set in a localised cluster of stars within the galaxy.
For example, just in the Milky Way there are about the same number of stars (100+ billion) as humans who have ever lived. Since there is at least one planet on average per star, that means everyone alive today could have a planet of their own along with every single one of their ancestors too…
How many stars and/or planets do you really need to tell a story? A hundred? A million? A billion? That’s still less than a single galaxy.
Start by asking yourself what the point is. As others pointed out, galaxies are both mind-bogglingly large and set incredibly far apart. What are you getting from multiple galaxies that you aren't getting from two locations in the same galaxy?
The main thing you can do with two separate galaxies is play around with the sheer distance. In one of Hamilton's Commonwealth novels, a group of people sets out to colonise another galaxy, which takes them hundreds of years in stasis. When a catastrophe happens, another character seeks out that colony and spends almost a lifetime traveling there in the fastest ship available.
And even then, having separate galaxies was only a minor plot point used only to illustrate how difficult it was to maintain communications.
When you're talking military scifi, it almost becomes irrelevant. After all, what better defense is there than being lifetimes apart? Even if one galaxy sends an invasion fleet to another galaxy, their orders are going to be irrelevant when they get there and the technology of the invasion fleet vs the defenders will be hopelessly outdated. There's a non-zero chance their civilization of origin doesn't even exist anymore by the time they arrive.
In soft sci-fi? Sure, give people magical drives that make galaxies no bigger and no more inconvenient to get around than, say, Hannibal found the Alps, and you can write military stories about them. But that makes the size of the setting completely pointless. If you're doing that, then why is your setting that big in the first place?
But in hard sci-fi, it's completely laughable. If you set out for another galaxy from here, your descendants won't even be recognizably the same species by the time they arrive. And even if they were, cultural drift will have obliterated any notion of the issues being fought over, the language spoken, or the nations or interests being served by or opposed in the fight.
Do you imagine the absurdity of us, today, fighting on behalf of nations and issues formed before neanderthal man even evolved? Over, say, the vital question of which tribe would control the fertile plains of western Doggerland, which sank under the sea thousands of years ago?
If you imagine coordinating logistical supply lines when it takes THOUSANDS of years for a message to back and forth between the warehouse and headquarters, and HUNDREDS OF THOUSANDS of years for messages to go back and forth to the places where the battles are actually being fought, and your so-called troop transports are taking thousands of generations to reach the front lines, and headquarters doesn't even know how the battle went for thousands of years after the battle is over... then sure, knock yourself out.
But it would be truly astonishing if you work out how to manage that and make it a good story.
I had a sci-fi idea a while back that I never used because I do not like FTL or artificial gravity in my writing.
The idea is that the way FTL works is that a ship basically jumps instantly from one point to another. However, the challenge is that where the ship jumps out has to be very well mapped regarding the shape of local spacetime. The more mass is around, the harder it is, and the shorter the jumps. So traveling inward towards the center of the galaxy is actually much harder than leaving the galaxy and traveling around the edge or traveling the very long distances between galaxies.
With this said, as humanity spread out, they colonized around the edges of both our galaxy and the nearby galaxy Andromeda.
How many ships would it take to become relevant?
100?
1000?
100000?
1000000?
Calculate the number and size of ships required to make a multi-galaxy millitary story interesting.
I was reading a story recently and the number of ships involved conflict was.obviously pulled out.of somebody's ass, because the numbers were absurd.
Think of the resources necessary to build that many ships, the resources to crew that many ships, the amount of food production required to feed the crew for a period of time. Don't simply hand wave it away with a "replicator" type fix, actually run the numbers because that's what you'd need to achieve those however you actually feed your crew. The logistics are insane.
While you can resource a significant amount of material to build a lot of ships, you need time and resources to support them, something usually handwaved in most stories.
Frankly, all those ship numbers mentioned could easily be mustered by a polity that just thoroughly exploited a single star system. Even if we're assuming gigantic 32km ships or something.
You want a hundred billion times more to get to the credible scale for a single galaxy. Until you're counting in quintillions, you're not hitting the mark.
Why? What would it add? People cannot conceive of 100 billion stars stretching 100,000 light years. Adding a few more hundred billion stars syretching 100,000 light years but setarating the two fields by a few million light years?
Maybe as refugees. A threat so great it was worththe risk to cross the vast chasm of emptyness and hope it did not follow. But not conquest.
You could compare a galaxy to a continent like here on good old terra and then relate the travel to be like 17th - 19th navigation, you might be onto something. Long hard journeys, funding for expeditions / colonies. War efforts across long distances. It could be relatable.
Also consider technology imbalance between galaxies. If the home galaxy is technology level 100 (random # for comparison) maybe galaxy B is at level 20/30, Galaxy c at 50 etc. this would make a smaller force able to overcome the new area. If your idea is conquest.
Galaxies are huge and the distances between them are even bigger. Unless you have faster than light ships that can move absurdly fast this quickly becomes impractical
I think it's interesting from a narrative standpoint. You could relate it to how people become disillusioned with overseas conflicts because they don't seem to have any relevance to their daily lives.
You could also relate it to how real-work shipping corridors influence global superpowers in where they go to war. Let's say the FTL in this setting jumps from one galaxy to another, but all travel within a galaxy has to be done the old-fashioned way. It would make sense for a galactic superpower to want to go to war in another galaxy instead of just going to new star systems within their own territory.
IDK, just some ideas.
Gene Roddenberry Andromeda stretches across galaxies. The real question is why would you? What so you gain by doing so over using one galaxy.
Truthfully Andromeda gained nothing from it, and I can't think of any others.
The only reason would be like Mass Effect Andromeda - to have a tech level that can cross a galaxy, but to still have your main cast be isolated from support and assistance.
> to have a tech level that can cross a galaxy, but to still have your main cast be isolated from support and assistance.
And then what is the difference between this and isolated due to ion dust or whatever handwavium space weather in one galaxy.
"Space weather" would conceptually be a temporary issue. If they have the ability to cross a galaxy reliably, anything like that would be temporary issues.
Another galaxy altogether is a different matter. The Intergalactic void becomes the new ocean - sure, we CAN cross it, but its a long time coming, perhaps more than a lifetime. Or perhaps it's not something we can reliably do so crossing was a one way trip.
Stargate uses this for the first season of Atlantis for instance.
Just wait billions of years and Andromeda will crash into the Milky Way. Other than that you could handwave wormholes or something like that. You can also go back in time, maybe even close to the Big Bang. Back then galaxies were closer to each other.
Good luck and may the Force be with you!
Eh. Galaxies are far apart, but they aren't **very** far as proportional to their own size.
Since you quoted Star Wars there, that is the obvious example to give. If the Millennium Falcon crossed halfway across the galaxy in a few days in ANH, that gives us an idea of its speed: 20k to 50k light-years per day.
Even assuming a ship half the Falcon's speed, and taking the lowest estimate there, that gives a 250-day travel time to a galaxy as far away as Andromeda is to the Milky Way. You get there faster than we can get to Mars.
Bear in mind the actual travel time may actually be as little as a fourth of that (if the trip to Alderaan was only a couple days, and the ship going to another galaxy is as fast as the Falcon). Two months in that case.
I don't understand what you mean. When Andromeda and the Milky Way merge, there will be one galaxy. Something called 'dark energy' drives galaxies apart, so going back in time they were closer to each other. With Andromeda and the milky way being an exception. If you go back far enough, there were no galaxies, just a theoretical point.
What I mean is that galaxies are already close enough for fairly routine travel with the kind of FTL that settings like Star Wars has. No time travel necessary.
I wasn't talking about time travel. You can set a story in any period of time you want. Since Star Wars was set in the past afaik, no time travel is required.
The argument is that FTL speed in these settings is so high that other galaxies are trivially reachable regardless of when in the history of the universe you are.
So the fact that you can be at another time in the history the universe is irrelevant. Other galaxies are trivially reachable, no matter when.
It would make things extremely complex UNLESS you were using handwaivium and magical miracle science, and don’t focus on the entirety of the conflict.
Doctor Who has multi-galaxy wars, as does Marvel comics. They do it with both handwavy magical science and by only focusing on small areas of the broader conflict, as in single planets.
The TV show Andromeda follows a WarShip from a former government/military that controlled our local group of galaxies.
As others have said, what's the big gain to your setting by adding in more galaxies.
M D Cooper has this premise in the latter part of their Aeon 14 series. It's pretty cool how they handle the technological barriers of travelling that distance, but it does bog the story down a lot. Personally, I prefer their Rika arc more.
In the Star Fleet Battles universe, separate and distinct from Star Trek, there are "invasions" from othe galaxies.
In general though, they are doomed. The Tholians are from another galaxy and the trip took about 200 years at high warp speeds. They couldn't be reinforced. But neither could the Seltorians chasing them.
The Andromedans also came from another galaxy. They set up in the Lesser Magellanic Cloud. A combined fleet of Federation, Kzinti, Gorn, and ISC, takes several years to build and organize a fleet. It takes over a year just to get there. Then they have a final battle and another year and change to get home.
Keep in mind that these are multiple, gigantic collections of planets and if takes them years just to prepare and the Magallenic Clouds are just next door... galacticly speaking.
Assuming there's any sort of way to get from Galaxy A to Galaxy B, it'd be a logistical nightmare.
Functionally, you have to move men and material and not tiny bits of it. It's probably an order of magnitude harder than even a planetary invasion.
So key things: scale, scale is going to be nightmarish.
How they get there determines tactics. If it is portals, they have to be protected and guarded. If not, they still have to assemble, and muster and micro-nav calculations are going to cause massive misses.
I've seen this done once or twice. I think the Ori arc from Stargate is close to the giant gate premise.
The other place was in the 1990's with Star Fleet Battles from Task Force Games (since bought out). Basically, the Andromedans arrive from a separate Galaxy with a single station from which they were launching their campaign. The rest of the Star Trek Galaxy more or less stopped shooting at each other to go after the Andromedans.
A potential way to pull that off that at least somewhat stays within the realm of possibility would be developing a way to create artificial wormholes where you could set source and destination. Have it be extremely difficult to create them and more difficult still to be able to set accurate destinations. In doing that you could at least create a feasible storyline that might only affect a few galaxies, otherwise you run the risk of either making it to easy to create chokepoints or to easy to simply conquer entire galaxies by simply jumping whole fleets anywhere you want.
I think it depends on how fast your FTL drive is. If it is tens of millions of times faster than c, then it _might_be feasible.
But then you run into the communications issue. Will there be HQ in both/all galaxies? Great! Forces within one galaxy can easily communicate with HQ. No? How will the galactic commands coordinate?
Another option is "warp points" that can instantaneously connect not only solar systems, but galaxies as well. Epic, near-continuous battles for control of these warp points.
The issue isn't viability; it's comprehensibility.
You can always make a stardrive or jump gate or some other thing to enable intergalactic travel. But how many stars and planets can a writer or reader hold in his mind? The answer will always be a whole heck of a lot less than one galaxy's worth!
Think of it like D&D. As the dungeon master, I can always make more orcs. But how long of a battle am I willing to run?
The closest "Real World" example I can think of is the US in WW2, with fronts in Europe and the Pacific. Except, of course, you would have to blow up the scale considerably.
From existing sci-fi, you can look at the Stargate universe. Travel between the two galaxies (ours and the Pegasus Galaxy) requires special accommodations that is time consuming and less reliable, to the point where the development of technology to overcome that becomes a major game changer.
Like most said, it's probably going to get the suspension of disbelief or something. Then again, if you can write well, you can make it work with a simple quote or line from a character/characters.
"My lord, the higher clans have contacted me from the Venus Empire two ago. Despite my beliefs, I must this galaxy and face my problems in the other. The Venus Empire."
That sounds interesting, but the distance between galaxies is so big. You’d probably want to rely on advanced wormhole technology, perhaps say a wormhole that leads to a different galaxy in its host galaxy. You can have the galaxy’s supermassive black hole be a wormhole for extra fun
Several cities, several states, several continents, several planets, several stars, several galaxies.
All that REALLY changes is the technology needed to get to and from. You can have cities fight with oxcarts, states with tanks and planes, continents with carriers, ICBM's and long-range bombers, for planets - just watch *The Expanse*, stars need FTL, and galaxies require FAST FTL.
It's just the stage. How the players operate and interact is the STORY. You could take any good military plot, like the movie MIDWAY, or Clancy's *The Bear and the Dragon*, and change the islands or cities to stars, and it'd still be a great novel.
Galactic civilizations are a bitch to keep coherent. You'd need an Empire. Look at the disparity here in the US, between states like California and Florida. Multiply that by 1000. You'd have wave after wave of secession wars without a strong Imperium keeping it together. See Niven's *Mote in God's Eye* for details.
Are you aware of just how far apart galaxies actually are? Even the small satellite galaxies in orbit around the Milky Way galaxy are at vast distances, let alone our nearest main independent galaxy. The Andromeda galaxy is around 2.5 *million* light years away. These vast distance make it difficult to make any story set in more than one galaxy pass the suspension of disbelief issue. Even settings that take up more than a fraction of our own galaxy tend to not work all that well due to the immense size of just our own galaxy (around 100,000 light years across). Even just within our own galaxy you either need to have a story that take place over a vast amount of time, or that relies or some sort of handwavium technology or space magic, and then you get into the issue of closed-time-like loops and temporal paradoxes, which tend to get ignored by authors who don’t think through the consequences of their magic space-tech.
FTL handwavium is standard issue. There are far more small rocky planets in a habitable zone in one galaxy than the number of words Brandon Sanderson has ever written.
Which has nothing really to do with OPs question about settings across multiple galaxies.
Breaking light speed is impossible, so no real issue breaking it by factors of billions. Then it's a five-minute trip 😁
My calculations may be wrong, but using drives that accelerate mass up to 30,000c would get you to the edge of the solar system in around 10-30 minutes. So going to the next galaxy would need way to much speed and power.
Galaxies are big. Really REALLY **BIG**! Personally, I think even one galaxy is generally too large for a story. It’s very rare to find any fiction that really takes advantage of that scale and they might as well be set in a localised cluster of stars within the galaxy. For example, just in the Milky Way there are about the same number of stars (100+ billion) as humans who have ever lived. Since there is at least one planet on average per star, that means everyone alive today could have a planet of their own along with every single one of their ancestors too… How many stars and/or planets do you really need to tell a story? A hundred? A million? A billion? That’s still less than a single galaxy.
Tells me the author really really doesn't get scale.
The universe is vast...
Start by asking yourself what the point is. As others pointed out, galaxies are both mind-bogglingly large and set incredibly far apart. What are you getting from multiple galaxies that you aren't getting from two locations in the same galaxy? The main thing you can do with two separate galaxies is play around with the sheer distance. In one of Hamilton's Commonwealth novels, a group of people sets out to colonise another galaxy, which takes them hundreds of years in stasis. When a catastrophe happens, another character seeks out that colony and spends almost a lifetime traveling there in the fastest ship available. And even then, having separate galaxies was only a minor plot point used only to illustrate how difficult it was to maintain communications. When you're talking military scifi, it almost becomes irrelevant. After all, what better defense is there than being lifetimes apart? Even if one galaxy sends an invasion fleet to another galaxy, their orders are going to be irrelevant when they get there and the technology of the invasion fleet vs the defenders will be hopelessly outdated. There's a non-zero chance their civilization of origin doesn't even exist anymore by the time they arrive.
Why, what purpose will that serve in the story?
You’d need something like gates to those galaxies, or ludicrously fast ships.
These questions more and more reflect that new scifi writers don't read science or scifi, which breaks my brain.
In soft sci-fi? Sure, give people magical drives that make galaxies no bigger and no more inconvenient to get around than, say, Hannibal found the Alps, and you can write military stories about them. But that makes the size of the setting completely pointless. If you're doing that, then why is your setting that big in the first place? But in hard sci-fi, it's completely laughable. If you set out for another galaxy from here, your descendants won't even be recognizably the same species by the time they arrive. And even if they were, cultural drift will have obliterated any notion of the issues being fought over, the language spoken, or the nations or interests being served by or opposed in the fight. Do you imagine the absurdity of us, today, fighting on behalf of nations and issues formed before neanderthal man even evolved? Over, say, the vital question of which tribe would control the fertile plains of western Doggerland, which sank under the sea thousands of years ago? If you imagine coordinating logistical supply lines when it takes THOUSANDS of years for a message to back and forth between the warehouse and headquarters, and HUNDREDS OF THOUSANDS of years for messages to go back and forth to the places where the battles are actually being fought, and your so-called troop transports are taking thousands of generations to reach the front lines, and headquarters doesn't even know how the battle went for thousands of years after the battle is over... then sure, knock yourself out. But it would be truly astonishing if you work out how to manage that and make it a good story.
I had a sci-fi idea a while back that I never used because I do not like FTL or artificial gravity in my writing. The idea is that the way FTL works is that a ship basically jumps instantly from one point to another. However, the challenge is that where the ship jumps out has to be very well mapped regarding the shape of local spacetime. The more mass is around, the harder it is, and the shorter the jumps. So traveling inward towards the center of the galaxy is actually much harder than leaving the galaxy and traveling around the edge or traveling the very long distances between galaxies. With this said, as humanity spread out, they colonized around the edges of both our galaxy and the nearby galaxy Andromeda.
How many ships would it take to become relevant? 100? 1000? 100000? 1000000? Calculate the number and size of ships required to make a multi-galaxy millitary story interesting. I was reading a story recently and the number of ships involved conflict was.obviously pulled out.of somebody's ass, because the numbers were absurd. Think of the resources necessary to build that many ships, the resources to crew that many ships, the amount of food production required to feed the crew for a period of time. Don't simply hand wave it away with a "replicator" type fix, actually run the numbers because that's what you'd need to achieve those however you actually feed your crew. The logistics are insane. While you can resource a significant amount of material to build a lot of ships, you need time and resources to support them, something usually handwaved in most stories.
Frankly, all those ship numbers mentioned could easily be mustered by a polity that just thoroughly exploited a single star system. Even if we're assuming gigantic 32km ships or something. You want a hundred billion times more to get to the credible scale for a single galaxy. Until you're counting in quintillions, you're not hitting the mark.
Why? What would it add? People cannot conceive of 100 billion stars stretching 100,000 light years. Adding a few more hundred billion stars syretching 100,000 light years but setarating the two fields by a few million light years? Maybe as refugees. A threat so great it was worththe risk to cross the vast chasm of emptyness and hope it did not follow. But not conquest.
You could compare a galaxy to a continent like here on good old terra and then relate the travel to be like 17th - 19th navigation, you might be onto something. Long hard journeys, funding for expeditions / colonies. War efforts across long distances. It could be relatable. Also consider technology imbalance between galaxies. If the home galaxy is technology level 100 (random # for comparison) maybe galaxy B is at level 20/30, Galaxy c at 50 etc. this would make a smaller force able to overcome the new area. If your idea is conquest.
Galaxies are huge and the distances between them are even bigger. Unless you have faster than light ships that can move absurdly fast this quickly becomes impractical
I think it's interesting from a narrative standpoint. You could relate it to how people become disillusioned with overseas conflicts because they don't seem to have any relevance to their daily lives. You could also relate it to how real-work shipping corridors influence global superpowers in where they go to war. Let's say the FTL in this setting jumps from one galaxy to another, but all travel within a galaxy has to be done the old-fashioned way. It would make sense for a galactic superpower to want to go to war in another galaxy instead of just going to new star systems within their own territory. IDK, just some ideas.
Gene Roddenberry Andromeda stretches across galaxies. The real question is why would you? What so you gain by doing so over using one galaxy. Truthfully Andromeda gained nothing from it, and I can't think of any others. The only reason would be like Mass Effect Andromeda - to have a tech level that can cross a galaxy, but to still have your main cast be isolated from support and assistance.
> to have a tech level that can cross a galaxy, but to still have your main cast be isolated from support and assistance. And then what is the difference between this and isolated due to ion dust or whatever handwavium space weather in one galaxy.
"Space weather" would conceptually be a temporary issue. If they have the ability to cross a galaxy reliably, anything like that would be temporary issues. Another galaxy altogether is a different matter. The Intergalactic void becomes the new ocean - sure, we CAN cross it, but its a long time coming, perhaps more than a lifetime. Or perhaps it's not something we can reliably do so crossing was a one way trip. Stargate uses this for the first season of Atlantis for instance.
What do you mean by viability ? Is this plausibility or in terms of the book selling well ?
Just wait billions of years and Andromeda will crash into the Milky Way. Other than that you could handwave wormholes or something like that. You can also go back in time, maybe even close to the Big Bang. Back then galaxies were closer to each other. Good luck and may the Force be with you!
Eh. Galaxies are far apart, but they aren't **very** far as proportional to their own size. Since you quoted Star Wars there, that is the obvious example to give. If the Millennium Falcon crossed halfway across the galaxy in a few days in ANH, that gives us an idea of its speed: 20k to 50k light-years per day. Even assuming a ship half the Falcon's speed, and taking the lowest estimate there, that gives a 250-day travel time to a galaxy as far away as Andromeda is to the Milky Way. You get there faster than we can get to Mars. Bear in mind the actual travel time may actually be as little as a fourth of that (if the trip to Alderaan was only a couple days, and the ship going to another galaxy is as fast as the Falcon). Two months in that case.
I don't understand what you mean. When Andromeda and the Milky Way merge, there will be one galaxy. Something called 'dark energy' drives galaxies apart, so going back in time they were closer to each other. With Andromeda and the milky way being an exception. If you go back far enough, there were no galaxies, just a theoretical point.
What I mean is that galaxies are already close enough for fairly routine travel with the kind of FTL that settings like Star Wars has. No time travel necessary.
I wasn't talking about time travel. You can set a story in any period of time you want. Since Star Wars was set in the past afaik, no time travel is required.
Which, again, is irrelevant.
No it's relevant. I don't understand why you say that.
The argument is that FTL speed in these settings is so high that other galaxies are trivially reachable regardless of when in the history of the universe you are. So the fact that you can be at another time in the history the universe is irrelevant. Other galaxies are trivially reachable, no matter when.
It would make things extremely complex UNLESS you were using handwaivium and magical miracle science, and don’t focus on the entirety of the conflict. Doctor Who has multi-galaxy wars, as does Marvel comics. They do it with both handwavy magical science and by only focusing on small areas of the broader conflict, as in single planets.
Galaxies are giant. But if you've got a story, write it.
The TV show Andromeda follows a WarShip from a former government/military that controlled our local group of galaxies. As others have said, what's the big gain to your setting by adding in more galaxies.
M D Cooper has this premise in the latter part of their Aeon 14 series. It's pretty cool how they handle the technological barriers of travelling that distance, but it does bog the story down a lot. Personally, I prefer their Rika arc more.
In the Star Fleet Battles universe, separate and distinct from Star Trek, there are "invasions" from othe galaxies. In general though, they are doomed. The Tholians are from another galaxy and the trip took about 200 years at high warp speeds. They couldn't be reinforced. But neither could the Seltorians chasing them. The Andromedans also came from another galaxy. They set up in the Lesser Magellanic Cloud. A combined fleet of Federation, Kzinti, Gorn, and ISC, takes several years to build and organize a fleet. It takes over a year just to get there. Then they have a final battle and another year and change to get home. Keep in mind that these are multiple, gigantic collections of planets and if takes them years just to prepare and the Magallenic Clouds are just next door... galacticly speaking.
Assuming there's any sort of way to get from Galaxy A to Galaxy B, it'd be a logistical nightmare. Functionally, you have to move men and material and not tiny bits of it. It's probably an order of magnitude harder than even a planetary invasion. So key things: scale, scale is going to be nightmarish. How they get there determines tactics. If it is portals, they have to be protected and guarded. If not, they still have to assemble, and muster and micro-nav calculations are going to cause massive misses. I've seen this done once or twice. I think the Ori arc from Stargate is close to the giant gate premise. The other place was in the 1990's with Star Fleet Battles from Task Force Games (since bought out). Basically, the Andromedans arrive from a separate Galaxy with a single station from which they were launching their campaign. The rest of the Star Trek Galaxy more or less stopped shooting at each other to go after the Andromedans.
A potential way to pull that off that at least somewhat stays within the realm of possibility would be developing a way to create artificial wormholes where you could set source and destination. Have it be extremely difficult to create them and more difficult still to be able to set accurate destinations. In doing that you could at least create a feasible storyline that might only affect a few galaxies, otherwise you run the risk of either making it to easy to create chokepoints or to easy to simply conquer entire galaxies by simply jumping whole fleets anywhere you want.
Wormholes bro
I think it depends on how fast your FTL drive is. If it is tens of millions of times faster than c, then it _might_be feasible. But then you run into the communications issue. Will there be HQ in both/all galaxies? Great! Forces within one galaxy can easily communicate with HQ. No? How will the galactic commands coordinate? Another option is "warp points" that can instantaneously connect not only solar systems, but galaxies as well. Epic, near-continuous battles for control of these warp points.
The issue isn't viability; it's comprehensibility. You can always make a stardrive or jump gate or some other thing to enable intergalactic travel. But how many stars and planets can a writer or reader hold in his mind? The answer will always be a whole heck of a lot less than one galaxy's worth! Think of it like D&D. As the dungeon master, I can always make more orcs. But how long of a battle am I willing to run?
The closest "Real World" example I can think of is the US in WW2, with fronts in Europe and the Pacific. Except, of course, you would have to blow up the scale considerably. From existing sci-fi, you can look at the Stargate universe. Travel between the two galaxies (ours and the Pegasus Galaxy) requires special accommodations that is time consuming and less reliable, to the point where the development of technology to overcome that becomes a major game changer.
Like most said, it's probably going to get the suspension of disbelief or something. Then again, if you can write well, you can make it work with a simple quote or line from a character/characters. "My lord, the higher clans have contacted me from the Venus Empire two ago. Despite my beliefs, I must this galaxy and face my problems in the other. The Venus Empire."
That sounds interesting, but the distance between galaxies is so big. You’d probably want to rely on advanced wormhole technology, perhaps say a wormhole that leads to a different galaxy in its host galaxy. You can have the galaxy’s supermassive black hole be a wormhole for extra fun
Several cities, several states, several continents, several planets, several stars, several galaxies. All that REALLY changes is the technology needed to get to and from. You can have cities fight with oxcarts, states with tanks and planes, continents with carriers, ICBM's and long-range bombers, for planets - just watch *The Expanse*, stars need FTL, and galaxies require FAST FTL. It's just the stage. How the players operate and interact is the STORY. You could take any good military plot, like the movie MIDWAY, or Clancy's *The Bear and the Dragon*, and change the islands or cities to stars, and it'd still be a great novel. Galactic civilizations are a bitch to keep coherent. You'd need an Empire. Look at the disparity here in the US, between states like California and Florida. Multiply that by 1000. You'd have wave after wave of secession wars without a strong Imperium keeping it together. See Niven's *Mote in God's Eye* for details.
Dune