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.
... is 3.32.
In the town the population is spread out with 24.7% under the age of 18, 7.4% from 18 to 24, 32.6% from 25 to 44, 23.2% from 45 to 64, and 12.2% who are 65 years ...