The Oldest Question in the Universe

And it was all going so nicely. Let’s Kill Hitler finally started providing us with a few answers and then Steven Moffat had to go and leave us hanging after this scene:

Doctor: “Who wants me dead?”
Teselecta: “The Silence”
Doctor: “What is the Silence? Why is it called that? What does it mean?”
Teselecta:  “The Silence is not a species; it is a religious order or movement. Their core belief is that silence will fall when the question is asked.”
Doctor: “What question?”
Teselecta:  “The first question; the oldest question in the universe. Hidden in plain sight.”
Doctor: “Yes, but what is the question?”
Teselecta:  “…Unknown…”

At least we got a funny line out of it though, “Well fat lot of use that is, you big ginge!”

So what do you think the question is?