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.
... 28.5% under the age of 18, 6.3% from 18 to 24, 32.3% from 25 to 44, 21.1% from 45 to 64, and 11.8% who are 65 years of age or older. The median age is 37 years. Fo ...