In short: you don't know what you don't know. And given that you do not know you don't know it, there's no way for you to ask specific-enough a question to get a specific answer back, from Google or for a friend. It is the theory behind the much simpler concept I came up with, of "Ungoogleable Thinking".
I write the Masters course at Charles Sturt Uni on Designing Spaces for Learning. The key concept above is described in this video clip. It's simple, and at the same time one of the most complex concepts for my students to get their heads around.
The key point made in the video is what my team and I have tried to show through our work with schools: as you cannot seek out the answers to questions you cannot ask, you need another way to 'bump into' those unknown unknowns. The only two ways to do this are chance (have someone tell you something you didn't know - but that means a lot of teacher talk to get to a few morsels of new stuff for a whole class) or you enter into a voyage of discovery - everything else is going to be stuff you know you don't know, or that you know already.
This is where a teacher can curate resources, and provoke learners, to such an extent that we can take a safe guess that students will bump into concepts that they didn't know they didn't know. And when this happens, the learner needs to connect this new concept to what they know before, thereby creating new understanding and knowledge.
This means that, while useful some of the time, the traditional "understanding by design" project is unlikely to ever facilitate deeper learning of the "unknown unknown" variety. Why? Because the teacher has defined the end point and an ever-convergent route of arriving there for the students.
In our design thinking work, we tend to look at much fuzzier problem areas, leading to multiple routes to several potential outcomes. Learning goals are not met at the end of the project, therefore - there are too many potential routes to showing understanding or problem-solving for even the most expansive rubric to be usable. Instead, success criteria are met during a much more predictable period of immersion, where the resources curated by the teacher are highly likely to help learners understand their prior knowledge (known knowns) and stuff they knew they didn't know, but can find out to help them answer key questions (known unknowns). A troubling provocation is often the launchpad for students to try and take prior knowledge and new ideas, to try and create something new.
It is only at the point of students making their own independent synthesis of the rich information they've gleaned, that a potential disjunction might be created, a point at which the student wants to dive deeper or off at a tangent to explore a much fuzzier area of their understanding of the world.
If ever you are seeking ways to help every student hit their zone of proximal development, then Hatchuel and Weil's C-K Theory is not a bad place to start (though you might need more than 28 minutes of reading and viewing to get it, and see how your practice might change thanks to it!).