You know every popculture reference of an alien civilization
Somewhere in the universe, there's another intelligent lifeform, with development at a stage roughly equal to our own. They make and share video recordings edited to be emotional, informative, inspirational, or entertaining, and you know and recognize all of it (translated to your native language, of course).
You just don't know where they are, what they look like, the overall story lines, and possibly a way to convince anyone else that this is a real thing.