The Question

The Question was considered by the Silence to be "the first question in the universe, hidden in plain sight." The Silence believed that on the Fields of Trenzalore, at the fall of the Eleventh, when no creature could speak falsely or fail to answer, the question that must never be answered would be asked.

The Silence were bent on killing the Doctor, since they believed him to be the only one who knew the answer to the Question and that there would come a time when the Question was asked.

Dorium told the Doctor that “silence will fall [when the question is asked]” was, in fact, a poor translation (DW: The Wedding of River Song). He revealed that the core tenet of The Silence was actually that the question must never be answered and that, in context, “silence must fall [when the question is asked]” was a better translation.

Dorium also revealed to the Doctor that the Question was "doctor who?".

It is currently unknown why the Question cannot (or should not) be answered, although the Question had been asked of the Doctor many times throughout his history (see The "Doctor Who?" running joke).