Pictures like this are why science fiction will never die:
That’s Galaxy NGC 5194 in the visible light spectrum, with a smaller galaxy nearby. It’s pretty, but more to the point it’s full of possibilities. Every little white dot that seems to be a spec of dust is in fact a star, and we can’t help but imagine that many of them have planets in orbit around them. This is just too endless an amount of possibilities not to want to daydream about them.
That said, it also gives something of an indication of why galactic empires are kind of silly. NGC 5194 is approximately 60,000 light years across (our own Milky Way is estimated between 100,000 and 180,000 light years across). Galaxies are simply more immense than our imaginations are capable of comprehending. Even if one grants faster-than-light travel, a drive capable of moving a ship 10,000 times faster than the speed of light would require 6 years to cross NGC 5194. To put this in perspective, Alpha Centauri, the closest star to the Earth, is about 4.3 light years away. A ship traveling at 10,000 times the speed of light would get there in a little under four hours. Also to put this in perspective, according to Memory Alpha, that’s faster than the Enterprise at Warp 9. I should note, though, that Star Trek was never consistent about this and the chart is practically a joke since Voyager had an episode where Warp 7 was at least 4 million times faster than the speed of light.
To cross one tenth of NGC 5194, at 10,000 times the speed of light, would take seven months. That’s significantly longer than crossing the Atlantic Ocean by sailboat. Except instead of having a small colony with access to vast resources, one would get to an entire planet with an entire planet’s resources at its disposal. So if an empire located in the center of one tenth of NGC 5194 wanted to subdue a planet on the edge of its territory of roughly equal technological prowess, it would have to commit thousands of ships to a three and a half month journey to have any hope of mounting a militarily significant force when it arrives at the planet. If some other planet, especially on the far side of empire’s territory were to cause trouble, those thousands of ships couldn’t possibly help. This was the basic problem which the Roman Empire faced, and a big part of why it crumbled. Granted, there was degeneracy from rich living, but the empire was simply not tenable. Rome was big, but it needed far too many troops to possibly police the entire area. A galactic empire would face a very similar problem.
And there’s the further problem with a galactic empire that one must ask why it’s bothering with all this conquest. The Roman Empire conquered in order to steal, especially resources which were not easily available near Rome, and to enslave the conquered. But people who can construct tends of thousands of faster-than-light battle cruisers can presumably build themselves washing machines, and they already have access to all the resources that are going to be found within a solar system anyway. It would be possible for them to conquer just on the principle of the thing, but that’s approximately the only reason they might want to conquer anyone else. At which point administration would become a huge headache. It takes an awful lot of soldiers to pacify a few billion people. Probably the best bet would be some sort of insectoid species which produces millions of children per generation; they could at least populate a world fast enough to not need to send a tenth the population of their native planet in order to occupy someone else’s.
Anyway, all of these are solvable problems if one needs an evil galactic empire for a story, but I think it suggests that keeping things smaller is more manageable; one of the great things about vast distances is it gives people a chance to actually be different from each other. And surely this is one of the great things about science fiction, especially in the modern world where every corner of the globe has a McDonalds on it—we can tell stories about people who are genuinely different from ourselves.