Earth’s solitary moon, Luna, lacks atmosphere and resources, other than limited water deposited at the poles and very modest mineral wealth. Its biological sentient population, which is more than 97% human, lives in an interconnected scattering of bubble habitats and sealed caverns.
Luna was first settled with military and scientific outposts created by various Earth nations in the 21st and 22nd centuries. Interest in these colonies waxed and waned, and over time almost all became independent entities, sometimes over the protests of the colonists themselves, during periods when finances were tight in the earthside nations sponsoring the outposts. Over this period, the colonies by necessity became more self-sufficient, growing food using hydroponics, then later adding vat-grown fungus-based raw materials to be used in food printers.
Efforts to use Luna as a penal colony in the 23rd century met with stiff Lunar resistance and were eventually abandoned. The few Lunar prisons that were constructed in this phase mostly were decommissioned, and former inmates who stayed joined the Lunar Alliance.
Newly-independent colonies joined forces, creating political connections as well as delving subterranean tunnels to enlarge and connect some of the closer habitats. By the early 24th century, nearly every lunar habitat had become part of the Lunar Alliance, a loose governmental organization that left most powers and services in the hands of the individual habitats, but that supported and encouraged pan-Lunar trade, transport, and law enforcement.
Luna has become the undistinguished and usually-ignored lesser sibling of Earth, reasserting itself mainly with the occasional high-profile scientist, technologist, or artist.