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.
... (1787) and Krusenstern (1805). Both, however, regarded it as a peninsula, and were unaware of the existence of the Strait of Tartary, which was discovered in 1809 by ...