The Dark Nebula consists of common Hydrogen Clouds forming a Nebulae Structure of three nebula combined into one by the effects of the Dark Matter. Dark Matter is very abundant there and forms an ellipse effect on the space around the nebula which hampers visual scans of the interior of the nebula (it tends to bend the light from stars around it) thus making it very hard to identify visually unless you already know its there. It also has an effect from the Dark Energy produced by the violent tidal energy fields of Dark matter interacting with the regular matter of hampering FTL travel throughout the nebula, which is why the Empire employs miniature Mass Relays to travel from system to system. The star systems are relatively unaffected by the nebula storms besides the occasional ion interference and thats easily rectified by Electromagnetic Counter Measures (ECM) in system. The density of the nebula structures themselves act as a shield from Infrared scans deep within the nebula by outside observation and even hamper direct visual observation between systems for the most part. Traversing it is more in line with trying to float your way through a Foggy Dark room from the center of the room. The Dark Nebula is 138 Lightyears in Diameter.