What is even more dangerous than not knowing. Have n't we all seen and met people who seem to be doing something grossly wrong and are totally oblivious of it. They are actually ignorant of their shortcomings. Successful people know where they lack and know when to take the back seat and let the people who know the way take the drivers seat. While, the ignorant ones forcefully keep driving and waste significant time and often miss the chance completely.
Here is my two cents to it:
Accept that no one know everything (including you). This is a humbling realization, but then, it empties you and allow you to accept knowledge freely.
Write down what you don't know. Once in a while introspect and jot down where you are ignorant and make it a point to fill up your gaps
Take help as sometimes the "obvious" may not be evident to the person concerned. And also help others to see their gaps without being intrusive or you will end up losing friends.
Actually, it is never about what you know. It is always about being aware of the gaps, being aware of what you don't know, which is the key to success.