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 (presumably his regeneration), when no creature could speak falsely or fail to answer, the question that must never be answered would be asked.

The Silence planned to kill the Doctor, since they believed him to be the only one who knew the answer to the Question. Their first attempt was trying to frame the Doctor for the destruction of the universe by blowing up his TARDIS, causing the Cracks. This would cause the Alliance to lock him up in the Pandorica. (DW : The Pandorica Opens, The Big Bang) The second attempt was to turn Melody Pond into a weapon of sorts, killing the Doctor by Lake Silencio. Both of these failed, although the Silence are presumed to believe the Doctor is dead. (DW: The Impossible Astronaut, The Wedding of River Song).

Dorium told the Doctor that “silence will fall [when the question is asked]” was, in fact, a poor translation, and that it should be "silence must fall". Dorium also revealed to the Doctor that the Question was actually "Doctor Who?". (DW: The Wedding of River Song)