I'm pretty sure that we now know this thanks to Donald Rumsfield...

At least, we can infer as much, if we concede (as we logically must), that there are "known knowns, and known unknowns, then, ergo, there must also be unknown unknowns.

Keep in mind that the concept of known unknowns and unknown unknowns pertains to matters of facts, circumstances, etc., and not to degrees of notoriety or celebrity --- a washed out contestant on American Idol or X-Factor may claim to be a now known unknown. An applicant who never made the cut to get an on-air audition will remain, in perpetuity, an unknown unknown, but this wasn't what we, or at least Don Rumsfield and I, were referring to...