It's a big nebula that is usually referred to as the Emu, the name originated from australian aboriginal cultures. One of the blotches is called The Coalsack.
To answer it more clearly, it's not part of the Earth's atmosphere at all, it's out there in the milky way
Wefeh t1_isbpgia wrote
Reply to comment by -BigMan in Milky Way core over La Palma [OC][1017x1600] astrorms by ruslan_merzlyakov
It's a big nebula that is usually referred to as the Emu, the name originated from australian aboriginal cultures. One of the blotches is called The Coalsack.
To answer it more clearly, it's not part of the Earth's atmosphere at all, it's out there in the milky way