Van den Bergh 142. Image courtesy: Richard Crisp.
A dark nebula is a relatively dense (up to 104 particles/cm3) interstellar cloud of matter whose dust particles obscure the light from stars or bright nebulae beyond it and give the cloud the appearance of a starless region. Such nebulae have the appearance of holes in the sky. They range in size from globules, a few thousand astronomical units across, to sprawling regions of starless darkness such as the Coalsack. Also known as an absorption nebula.