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.
... (KADU) was founded in 1960, to challenge KANU. KADU's aim was to defend the interests of the small minority tribes, such as the Kalenjin[?] to which Moi belonged, ...