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.
... African American, 0.08% Native American, 1.62% Asian, 0.08% Pacific Islander, 5.94% from other races, and 2.43% from two or more races. 16.62% of the population are ...