An aggregation of dust grains in the space between stars. Interstellar dust absorbs,
scatters, and polarizes the light from distant stars, causing the interstellar extinction.
Large dark regions in the plane of the Milky Way and other galaxies are caused by
intervening clouds of dust. The conclusive proof for the presence of a general and
selective interstellar absorption is due to R. J. Trumpler (1930).
