Coalsack is a term used to describe a dark, cloudy patch in the southern sky, which is caused by the absence of stars. There are several synonyms for the word coalsack, including dark nebula, dust cloud, and starless void. These terms describe areas of space that are dark and devoid of stars, which can be quite striking against the bright, star-filled sky. Other synonyms for coalsack include black hole, celestial void, and cosmic abyss. While these terms are often used interchangeably, they all refer to the same phenomenon in space, which can be fascinating and mysterious to observe.