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.
... 11.00% African American, 1.00% Native American, 1.88% Asian, 0.00% Pacific Islander, 7.00% from other races, and 4.62% from two or more races. 14.38% of the population ...