Van den Bergh 142. Image courtesy: Richard Crisp.
A dark nebula, also known as an absorption nebula, is a relatively dense (up to 104 particles per cubic centimeter) 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.