As we learn, each of us has absorbed a vast store of knowledge about our world. Much of this is from our formal education, and much from our experiences (the school of hard knocks). From this, we have formed a world view which influences what we do daily, our view of society, and our "domain space". We may be experts in one "domain space" but novices in another.
From Wikipedia, a world view is "...the fundamental cognitive orientation of an individual or society encompassing the entirety of the individual or society's knowledge and point of view. A world view can include natural philosophy; fundamental, existential, and normative postulates; or themes, values, emotions, and ethics." Basically, your point of view distorted by your experiences or projected onto reality....
The sum total of our knowledge can be viewed as a very small sliver in a pie chart, that of "all knowledge". This small sliver is "what we know", the sum of your formal education, experiences, etc.
A bit larger slice of the pie chart is "what we know we don't know", for example knowing that someone understands the inner workings of cells or the math behind the Theory of Relativity, but that is not your domain.
However, most of the pie chart is "what we don't know that we don't know". For example, um, I'm not sure, but something about the details of life on Europa?
During our lives, we should strive to expand the two slices of pie to continually expand what we know and recognize more and more of what we know we don't know. This will chip away at the what we don't know we don't know, but there is not enough time in our lives (or the cumulative lives of mankind) to eliminate the third slice of the pie. All we can do is chip away at it.
The bottom line: learning is a lifetime task.