What is any one of the spaces in the Milky Way which are very black, owing to the nearly complete absence of stars; esp., the large space near the Southern Cross sometimes called the Black Magellanic Cloud called?

coalsack — Any one of the spaces in the Milky Way which are very black, owing to the nearly complete absence of stars; esp., the large space near the Southern Cross sometimes called the Black Magellanic Cloud. (Webster's 1913.)