The Farsiders are descended from humans placed on the Moon by the alien Preservers (loosely similar to Marvel's Celestials). Farsiders possess advanced technology compared to Terrans, and some of their bloodlines (particularly the Royal Family of Farside City) have the genetic potential for psionic powers such as telepathy and telekinesis, although these abilities have been fading out for many generations, presumably due to inbreeding. Farsiders tend to be taller and thinner yet physically weaker than Terrans (their artificial environment has about half Earth's gravity), with the exception of Mentac, who was raised on Earth. The Farsiders' culture is extremely conservative and isolationist, remaining practically unchanged for millennia, but they have in recent decades made an effort to learn more about the Terrans, since Terrans are now advanced enough to pose a potential threat to the Farside society.
A generation ago, this fear of the outside led to a drastic change in the Farside community. Selene, who would later come to be known as Lady Luna, stirred up the population's fears and overthrew and killed most of the Royal Family. Only one young prince was spared, by secretly escaping to Earth to be raised as a Terran. The boy would grow up to become Mentac, the father of the Atom Family. However, Lady Luna was able to gain possession of a legendary artifact-- the Moonstone, a device that could amplify her latent psionic power until she was the most powerful being in Farside City. Exulting in her power, she ruled over her people with an iron fist.
Selene's ambition was not satisfied by ruling everyone she'd ever known, however; not when there was a whole world of other potential subjects hanging in the sky, mocking her with their freedom. But Earth was not undefended; The Atom Family, including her nephew Mentac and his children, foiled her plans of conquest time and again, and eventually led a rebellion that overthrew her, much to her rage. Worse yet, she discovered that the Moonstone responded to the commands of Chase Atom more readily than to her.
Though Farside City is now free and under a more equitable government, it still must be vigilant, for Lady Luna still plots to take back power. The city's existence is known only to a few on Earth, but many superheroes have close relations with the Farsiders.
Outside of the city proper, but still within the artificial atmosphere, is the preserve of the Crater-Apes, genetically engineered solar-powered Gorilla-like creatures bred for labor. With no experienced Mentalists left alive, they have proven too difficult to control and have been put into isolation. Lady Luna has used them in her schemes in the past. They are analogous to the Alpha Primitives of the Inhumans.