Coalsack

or Coal Sack

[ kohl-sak ]
/ ˈkoʊlˌsæk /

noun Astronomy.

a dark nebula in the southern constellation Crux, whose dust particles obscure light from Milky Way stars behind it.
Also called Southern Coalsack.

Origin of Coalsack

First recorded in 1625–35; coal + sack1

British Dictionary definitions for coal sack

Coal Sack

noun

a dark nebula in the Milky Way close to the Southern Cross