The Coalsack is probably the most prominent the dark nebulae visible to the naked eye. It casts a dark silhouette against the Milky Way’s bright stripe of stars in the southern sky. The Coalsack is located roughly 600 light-years away in the southern part of the constellation of Crux (the Southern Cross).
This apparently starless dark patch is really an opaque cloud of interstellar dust blocking the light from the background stars. Dust grains in the cloud redden the starlight that reaches us by absorbing blue light preferentially, so that the red stars shimmering in the northern and darkest part of the Coalsack appear deeper red than they would in the absence of this dust.
Image Credit: ESO