A cloud of interstellar gas and dust that absorbs light from behind, so that it appears black against a brighter background. The absorbed light heats the dust particles, which then reradiate some of the energy as infrared radiation. Some of the background light is not absorbed but is scattered, or redirected. The Horsehead Nebula in Orion is a well-known dark nebula; another is the Coalsack in the southern Milky Way.