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.
... town the population is spread out with 20.6% under the age of 18, 5.0% from 18 to 24, 26.8% from 25 to 44, 27.7% from 45 to 64, and 19.9% who are 65 years of ag ...