The Coalsack Dark Nebula (or simply the Coalsack) is the most prominent dark nebula in the skies, easily visible to the naked eye as a dark patch silhouetted against the southern Milky Way. The black cloud of gas and dust is located in the Crux constellation and overlaps somewhat into the neighbour constellations Centaurus and Musca. Spanning some 60 light-years, the Coalsack is located at a distance of approximately 600 light years away from Earth and has an estimated mass of about 3,500 times that of the Sun. Despite its darkness, the Coalsack is not totally black; it has a very dim glow (10% of the brightness of the surrounding Milky Way) which comes from the reflection of the stars it obscures.

In Star Trek original series episode "Let That Be Your Last Battlefield", the planet Cheron is located near the Coalsack. It once supported a humanoid civilization of Cheron natives. The USS Enterprise was commandeered by Bele of the planet Cheron, who attempted to divert the Enterprise on a course which would have taken them straight to the Coalsack.

Recent observations have detected hot gas on the perimeter of the Coalscak, indicating that it may be within a region of active massive star formation and supernovae known as the Upper Centaurus-Lupus super-bubble. This region has produced large hot stars that burn out quickly and die explosively, sometimes heating interstellar gas to high temperatures.