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.
... Native American, 2.13% Asian, 0.05% Pacific Islander, 3.49% from other races, and 1.95% from two or more races. 11.66% of the population are Hispanic or Latino of any ...