Galactic empire
Galactic empires are a common trope used in science fantasy and science fiction, particularly in space opera. Many authors have either used a galaxy-spanning empire as background, or written about the growth or decline of such an empire. The capital of a galactic empire is frequently a core world or home world.
The best known to the general public today is probably the Galactic Empire from Star Wars, which was formed in turn from the Galactic Republic.
Most of these galaxy-spanning domains depend on some form of transportation capable of quickly or instantly crossing vast cosmic distances (usually measured in light-years), many times faster than could photons at light speed. These invariably require some type of propulsion or displacement technology forbidden by Einstein's Theory of Relativity, or that otherwise relies on theories that circumvent or supersede relativity. (See: warp drive; hyperspace; Alcubierre drive.)
The term "galactic empire" has, no doubt because of association with the Empire from Star Wars, gained an unfavorable reputation. However, the Galactic Empires from Foundation and the CoDominium universe are relatively benign organizations; indeed, much of the plot of the Foundation series revolves around the issue of who can best and quickest revive the fallen galactic empire, it being taken for granted that this is a positive and worthy aim.