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.
... White, 0.84% African American, 0.13% Native American, 1.89% Asian, 0.03% Pacific Islander, 0.62% from other races, and 1.04% from two or more races. 3.33% of the population ...