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.
... in the town. The population density is 1,533.8/km² (3,985.3/mi²). There are 1,784 housing units at an average density of 485.1/km² (1,260.4/mi²). The ...