The Coalsack is a dark nebula located in the southernmost part of the Milky Way Galaxy. The planet Cheron is located near the Coalsack. By the late-2260s, the region surrounding the Coalsack was yet uncharted by the Federation.
In 2268, when Spock had to speculate about the nature of the (yet unidentified) space amoeba encountered in Sector 39J, he remarked the amoeba was not a galactic nebula like the Coalsack. (TOS: "The Immunity Syndrome")
In 2269, the USS Enterprise was commandeered by Bele of the planet Cheron, who attempted to divert the Enterprise on course 4-0-3 mark 7 from their position near Ariannus. Chekov remarked that the course would have taken them straight to the Coalsack. (TOS: "Let That Be Your Last Battlefield")
The Coalsack is located approximately 600 light years away from Earth.