Blackball

A blackball, also known as a black world, is a planet, moon, or asteroid that has been blacklisted from government records and civilian travel. Travel to and from is refused unless specifically granted by authorities. The reasoning for this can come down to a wide variety of things, including quarantine, criminal activity, dangerous inhabitants, inhospitable environments, or, most often, politics. As far as most textbooks and civilian-accessible documents and historical records are concerned, blackballs simply do not exist. While they may be found on a homebrewed starmap, any governmental records will simply neglect to mention them - and, as most starmaps are built directly from humanity's government records, the common man will never know they exist.