Who are the Spacers? They are the descendants of a dozen or so outsystem colonies of the Inner Sphere that were first settled by humanity in the earliest days of FTL. Unlike the insystem colonies of Mars, Ganymede, etc; these colonies were planned, built, and populated by the best and the brightest on the worlds showing the most potential for terraforming and habitation. Since it was in the earliest days of FTL, the first ships took years, even decades to reach their destinations and once they arrived they would be cut off for years or more from Earth (this was well before any FTL communications).
What they built was extraordinary. While gene manipulation and tailor-made children fell into disrepute on Earth other than for medical reasons, on the Spacer worlds it developed beyond what anyone had forseen. Like Japan of the 19th Century, the Spacers were eventually fairly homogeneous in culture, ethnic background (an unlikely mix edging toward a Polynesian look on many of the worlds), and philosophy. Since they had to survive together their cultures tended toward a socialist utopian outlook, built on the backs of millions of robots and androids running purely automated factories and farms for net export.
Unlike Earth, which grudgingly employs robots and the other colonies where they are too expensive or simply can’t be maintained, Spacer worlds use them to excess. There is no poverty on Aurora for instance. It is unheard of. Everyone is an artist, engineer, scientist or the like. Obviously a low population helps, but with every need provided by the government (which are generally nearly pure democracies for internal affairs) most people’s lives are pleasant with estimated lifespans of 200 years or more with the newest generation expected to live even longer.
The Spacers are paranoid of what they have and protect it. They tried to break away from Earth peacefully, but EarthGov wouldn’t let them go. The war created massive animosity between the rest of humanity and these worlds. Millions of Spacers and Earthers died in the conflict which the Spacers won. Even more Spacers abandoned their homeworlds, unwilling to fight Earth. For over 50 years after the victory the Spacers controlled trade while Earth rebuilt it’s fleet. Intermittent conflicts continued; however, the discovery of alien species and the advent of the Outer Sphere colonies has thawed this animosity. These new colonies needed Spacer goods which are much closer than Earth the the affiliated Inner Sphere colonies; while the spacer worlds needed markets. More strife comes from overpopulated colonies that see the sparsely populated Spacer worlds are potential migration points. Fighting illegal immigration is a potential problem each Spacer world would have to address if it wasn’t for controlling their own Stargates.
A Spacer is generally taller and far healthier than the rest of human average. They are fairly social among their own kind but are somewhat paranoid of outworlders due to the disease, crime, mutations and cultural pollution potential of the rest of Humanity.