The Badlands are an area of space where the universe suffers from acid reflux disease.
The Badlands are a section of the Milky Way Galaxy in the Alpha Quadrant that is made up of very chaotic space. It borders the Federation and Cardassian space. The conditions are such that even the best Star Trek ships have difficulty operating in this area. There seems to be a high level of spacial anomalies and unusual radiation that permeates the area.
Because of the volatile nature of the Badlands it is often used as a place to hide by various factions in Star Trek, most notably the Maquis.