ASR Worldbuildin' Summary

The "Central Planets" / "Central Planetary Confederation" / "Centralists" are an empire which expanded from Earth; authoritarian and conservative, it's based on preserving what they had. Space Russia in the space cold war. Collioqually the 'Old Planets' maybe, with Earth as its 'Cradle'.
The "United Worlds" / "League of United Worlds" / "Unionists" are an empire made from colonists exiled from Earth, and is more progressive and diverse but still a colonising force and let's be real colonisation is the worst. Space USA in the space cold war.

Blank writes:

When humanity made its first steps to truly live amongst the starts, it was in bits and pieces. There wasn't a great plan of conquest under a single empire, not yet. People simply found their own places and with such distances involved who was going to tell them no? To a cartographer it was messy; there are no natural borders and humans have always had difficulty with three dimensional star maps. So if you wanted to make sense of it all, what you really needed to look at is the second most important world after Earth: a cosy planet named Arcadia. A planet just right for human life with a pre-estabilish alien ecosystem of it's own. If you were lost in the galaxy you could ask 'am i closer to earth or arcadia?'. This made a lot of people who felt Earth should remain the center of the universe very mad.

There's more to the reasons why but this establishes the whereabouts of the lines drawn in the first intergalactic revolutionary war (Humans hadn't reached other galaxies yet but were perfectly willing to fight to decide who owns them). The result was roughly half of the human populated worlds breaking ties with the homeworld and organising themselves as the League of United Worlds to protect their independence. The alliance of old world powers fighting against the so-called unionists became known as the Central Planetary Confederation. At the very least the Centralists managed to unite the homeworld, even if they did so with an authoritarian state waving about old traditions in dire need of protecting.

Space is a big enough place that one would sound very foolish to state it ain't big enough for the two of you yet that is exactly the situation these two super powers are in. What they don't like to admit is that their grip on the many distant planets is tenuous at best and many of them are still far more concerned with dealing with the aftermath of the war than ideological loyalty. The theory goes that if one planet realises it can just switch allegiance to get a better deal for itself, then the next planet might realise it can too, or god forbid start it's own third faction. This might lead to treating the far flung worlds as if they're actually important, and both sides would rather just throw a lot of scary ships at the problem then let that happen.

But one thing that breaks the illusion of an all powerful central or united government? Space pirates doing as they will untouched by the law.