A back-story is a literary device[?] often employed in order to lend a story depth or verisimilitude. Characters or events related to a story may only be given cursory treatment so as not to distract from the central plot development.
Probably the most extensive back-story ever created was that created by J. R. R. Tolkien for his Middle-earth stories.
... Demographics
As of the census of 2000, there are 3,059 people, 1,181 households, and 818 families residing in the town. The population density is 81.3/km² ...