Dark rooms (better called dark levels, please excuse my mistake) are a special kind of floor whose main caracteristic are rooms deprived of any candels or chandeliers making them completely dark despite a few weak illumination sources like gunfire, the enemies eyes, explosions, projectiles, explosive barrels, and a group of jellyfishes that only appear on these kind of levels (please note that it is not necesary to kill these jellyfishes; you will be allowed to leave the room even if they are still alive).
The jellyfishes drop loot like any other enemy but also provide illumination. Killing them is recomended onlywhen in possession of a good ilumination source(ie. Grenade Launcher, M14, Hanz's Pistol, etc.) or when in extreme need of money, health, or any other consumable.