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.
... people, 1,007 households, and 753 families residing in the village. The population density is 529.3/km² (1,369.6/mi²). There are 1,031 housing units at an ...