One way of building background is to have the characters refer to things without explaining exactly what those things are. The slight confusion caused is balanced by the sense given of a larger world, outside the plot. If a story is extended to a long enough series (especially when there's a high turnover of writers), most of these cryptic references are explained and/or used as Canon Fodder, but often one or two will never appear in story. What such references really mean is a favorite subject for fan-clubs. The rise of the web has reduced the incidence of such things (creators get asked), though not totally eliminated them.