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 panned to kill the Doctor, since they believed him to be the only one who knew the answer to the Question.

Dorium told the Doctor that “silence will fall [when the question is asked]” was, in fact, a poor translation, and that is should be "silence must fall".

Dorium also revealed to the Doctor that the Question was "Doctor Who?". (DW: The Wedding of River Song)