The Intergalactic Underworld is a vast association of criminals that operate in outer space, particularly in the Citadel Empire. This association of criminals has been together at least since the 15th century. The Intergalactic Underworld hosts an annual auction called the Annual Intergalactic Underworld Auction, which is used to "black market" items to criminals, such as Killer robots and death rays.
The Citadel, located in the Vega system, is a major location for the Intergalactic Underworld. The planet Vega particularly, which is also the Citadel Homeworld, is considered a hub for the Intergalactic Underworld, which is most likely illegal.