If one were to think this through in a speech-act kind of way, saying you don’t know something can be considered a performative utterance: just by saying it (or thinking or writing it) you now know of it’s existence. In an incredible way, the very declaration of its nonexistence brings it into being. Sort of like this famous example: a university president announcing “our doors are now open to [hitherto excluded group]” becomes more than just speech. It ripples through ‘the real world.’ Its validity converges at the moment of speech, when the mechanisms that have allowed that utterance to be possible at all (say, a change in admissions policy) and its future validity (that the public now knows about it) meet.

At the same time, how much value do we place in acknowledging the possibility of unknown unknowns? At most, its a tentative border drawn around a big empty nothingness. In military intelligence terms, maybe it would be something like, “the total of all strategies of groups we don’t know about, that we can imagine to be possible.” So what you have is nothing becoming something which become infinity (as in an mathematically sublime and unmanageable amount of possibility), that functions almost like nothing. Square one.

But of course, as you points out, what might be scary for Donald is kind of exciting for most of us. It’s amazing to suddenly hear a band you know absolutely nothing about, rather than (and this is reverse vicariousness again), one you don’t really listen to but “know of.”