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.
... in the town is $60,209, and the median income for a family is $67,375. Males have a median income of $50,715 versus $34,301 for females. The per capita income for ...