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.
... makeup of the town is 40.00% White, 48.41% African American, 0.30% Native American, 3.41% Asian, 0.00% Pacific Islander, 4.15% from other races, and 3.73% from two or more ...