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.
... racial makeup of the town is 89.82% White, 1.47% African American, 0.20% Native American, 1.45% Asian, 0.02% Pacific Islander, 3.86% from other races, and 3.17% from tw ...