Mirkwood, which was known before for a long time as Greenwood the Great, was the greatest of the remaining forests of northern Middle-Earth and was very old. But already some 2000 years before the Dwarves and Bilbo came there it became an evil place, due to Sauron, which was known later, who founded a stronghold in the south of the forest, Dol Guldur. It became a dangerous place haunted by his creatures except a small part in the west where Woodmen dwelt and in the north-east, where was a kingdom of the Wood-Elves which also didn't have friendly feelings for Dwarves.
