These are those concepts that we do not know that we are not aware of. Borrowed from the Johari window, this phrase talks about the facts that we do not have any inkling about and are not suspecting. This phrase was used to speculate about the presence of war weapons with Iraq about which the US had no inkling.
Diagrams
Originated from/Coined by
Donald Rumsfeld
Further Reading/ References
https://en.wikipedia.org/wiki/There_are_known_knowns
https://hbr.org/2017/10/simple-ways-to-spot-unknown-unknowns
https://medium.com/lightspeed-venture-partners/knowns-vs-unknowns-78b0da5ca887