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.
AND STAR CLUSTERS