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.
... There are 6,949 housing units at an average density of 546.4/km² (1,414.4/mi²). The racial makeup of the town is 93.53% White, 1.36% African American, 0.15% ...