Xilmir

For the record, the principle I’m referring to here is a specific (and simplified because this is fiction, not a textbook) case of Noether’s first theorem, which is probably my favorite mathematical theorem. This is important because it means that I am the kind of person who has a favourite mathematical theorem.
Not only that, I’m the kind of person who had to consider for a moment whether or not I actually preferred the Folk theorem.
This is another world I may come back to. One post seems kind of inadequate for a setting the whole point of which is that it is far too large, and too diverse, to meaningfully describe.
For the record, it is significantly bigger than our observable universe. An empire controlling ten or twenty galaxies would be an insignificant portion of our universe, but it would at least be large enough to measure as a portion. It would show up on your calculator screen. This is a world in which an empire spanning ten or twenty thousand ‘galaxies’ is still small enough to be almost entirely obscure.
Yes, I am aware how large even a single galaxy is. When I say things about 'not knowing what you'll find', I bloody well mean it.

It is a fundamental mathematical principle that in any world with unchanging physical laws, energy will be conserved over time. And, for the most part, it is. But in many worlds, there is an exception - magic. Indeed, in many worlds, magic is magic because it breaks this fundamental principle - there not only does not, but can not exist any constant set of physical laws to describe the behavior of an isolated system in which magic is happening, because the energy of such a system is not constant.
This, as has been noted, is true on many worlds. But in few places is it more intimately weaved into society than on Xilmir. Because on Xilmir, wizards pay their taxes in lightning.
Every source of energy, save magic, requires the exhaustion of some resource or other. Thermodynamics is a harsh mistress - however efficiently a machine may be powered, some of the energy used to keep it going will end up as waste, impossible to recycle. And so, little by little, every mundane resource has been used up. The planets have been mined dry, and the stars have burned out - indeed, in recent years, many of the stars have been mined out. And yet, the world of Xilmar is thriving with life. It is a place where nations can span not only across planets, but across entire galaxies. Because magic does not care about the conservation of energy, and spits in the face of the ‘heat death of the universe’.
Almost every one of the myriad people of Xilmar know some magic - if for no better reason than that in a world where technology is almost universally powered by magic, it is often more convenient than finding somewhere to plug something in.
But there are ordinary citizens, and then there are wizards, capable of calling forth vast energies with no more effort than it might take most people to snap their fingers. And wizards are woven into every aspect of life on Xilmir, because they have to be - it is they who are the difference between civilization, and the end of everything. It is them upon whom many of Xilmar’s technological marvels rely, it is they who allow ships to fly at superluminal speeds, they who are able to rewrite the very nature of reality when it is convenient. And it is for this reason that wizards, regardless of what other differences might exist between civilizations, tend to be held in very high regard. In many places, indeed, they are the rulers.
Xilmar should, by all rights, be a rather boring world. The entire universe is explored. There are no horizons left. Science has advanced beyond what other worlds could dream of, technological progress has slowed to almost a complete halt simply because there is so little left that has not been tried, and even magic is about as advanced as it is ever going to be. There is even a near-complete lack of war, because there is so little to fight over. There are no natural resources left, what empires need, they can simply make. Ultimately, war is simply no longer worth it.
But the thing is, though all the universe may have been explored in a general sense, an explored universe is not really anything like an explored planet. You might not know everyone on your planet, but it is at least possible to have a general idea of the kinds of things that might be going on.
An entire universe isn’t like that. It’s simply too big. Even were one to spend only a few seconds on each one, one might spend centuries studying the wonders of the universe, and still be entirely assured of being able to fly to a random point in space, and find something entirely new. There are planets that have been made into spaceships, and spaceships constructed entirely from scratch that make planets look like grains of sand. There are species which are so closely blended with machines that it is impossible for even them to tell any more what is natural, and what is not. And there are species which think that such a thing is blasphemous. There are cretures made of pure energy, and wizards capable of feats that those only a few galaxies away would swear entirely impossible - even things that other wizards can definitively prove are impossible.
Xilmar is peaceful, but only on from a long way away - there are more individual dramas than there are numbers to count them. And, whilst with the size of the population involved, any quantifier would be almost meaningless, it is common for people to spend their time exploring the vastness of it, toppling tyrants and writing wrongs. Because with a whole universe to choose from, there is no shortage of either.