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.
... of 302.4/km² (783.9/mi²). The racial makeup of the village is 89.17% White, 4.63% African American, 0.42% Native American, 1.16% Asian, 0.05% Pacific Islander, ...