The Dark Sense mission senses most planets within 200 light-years that have been colonized by other races, along with their mineral information (combined in and on planet), MegaCredits and whether or not the planet has a starbase.
Planets that have been colonized by the Rebels will not be sensed, as they're inherently immune to the Dark Sense. In addition, planets that have not been colonized will not be sensed.
See also: Out of Fuel.