What's something that, when you include it in your world, makes everyone else in the world feel like it's "complete?"

I guess an example of this for me is the concept of the "Verse" and how the title of the show is always like "Stranger Things 4, Chapter 1: The Mystery of...". The idea of the show being set in a larger "Verse" gives each show a clear sense of being a part of a larger whole and I think that gives the show a sense of...completeness. <br><br>I guess another example would be the idea of an "official history" of your world. I've seen a lot of D&D campaigns and settings where the DM will create this really rich, detailed history for the world and that really grounds everything and gives it a sense of legitimacy and completeness. I think it's interesting to consider what other things, when added into your world, would do the same for your readers.