The Citadel is surrounded by a blue-tinted reflection nebula. The light of the nebula is actually light from the Citadel, scattered and reflected back at the station.
At first, the Serpent Nebula was assumed to be made of microscopic construction debris. Prevailing theory holds that the Protheans used molecular nanotechnology to manufacture the incredibly durable materials used in the Citadel. But unlike other nebulae, the Serpent does not dissipate over time. Therefore, it must be replenished constantly. The current popular theory is that the non-recyclable waste collected by the Citadel's keepers is somehow rendered down to the atomic or molecular level, and ejected into the cloud.
The thick nebula presents a navigation hazard. Beyond the relatively clear areas around the Citadel, electrical discharged are common. These are not blocked by kinetic barriers, and can severely damage metal-framed starships. In addition, some dense knots of dust can overwhelm the repulsion of kinetic barriers on smaller ships. If such a vessel is moving fast enough at the time, the effects are similar to being hit by a sandblaster.
Attempting to reach the Citadel through open space navigation is unadvisable; the only safe approach is through the various mass relays that orbit it.