The phrase "unknown unknowns" refers to things that we don't know that we don't know. It describes information or situations that we are completely unaware of, making it difficult to prepare for or predict them.
Sentences with «unknown unknowns»
But there are also unknown unknowns — the ones we don't know we don't know.» (voxfelina.com)
Sternberg Press published Kurant's monograph Unknown Unknown in 2008. (museumpublicity.com)
I seem to be at a stage in my reading where so many unknown unknowns are swiftly becoming known... (readinasinglesitting.com)