I'm currently teaching some college kids a class, and on Monday I had to explain about eight different things that they had never heard of or knew anything about (not fully related to the course material, but "general knowledge" things). I kinda felt bad.
(mentioned something) "Has anyone heard of this?" (lots of head shaking)
(,mention something else) "No one's heard about this, either"
(things like: how you fish for tuna fish, what Humboldt Squid are, etc. etc.)