What doesn't The Doctor know?

The Doctor goes into the future, the past, the present but is there such a thing as the unknown and could The Doctor fathom his way through as such?

I'd love to see a monster called 'The Unknown'. A monster literally nobody knows about, which didn't exist before, during or after and has the ability to make anyone it encounters unknown by everybody else with not even the victim's family knowing about the victim.

You are brave

The unknown, scary stuff but scary to The Doctor, surely not.

Clara- Doctor, something unknown?
The Doctor- Yes, what shall we do?
Clara-You ask me, do you not know?
The Doctor-Of course, you had to ask me so that I would know.
Clara- Doctor, you talk in riddles sometimes.

Then again, If they were called the Unknown, The Doctor found out about them and they became The Known but there was also The Silence and they couldn't be remembered for long, so The Unknown may become The Forgotten and then what would happen?

This?

http://static.comicvine.com/uploads/...forgotten2.jpg