The Question

The Question was "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 can speak falsely or fail to answer, the question that must never be answered will be asked."

The Silence planned to kill the Doctor, whom they believed the only one who knew the answer to the Question. (DW: Let's Kill Hitler) Their first attempt was to frame the Eleventh Doctor for the destruction of the universe by blowing up his TARDIS, causing the Total Event Collapse. This made the Alliance lock him up in the Pandorica. (DW: The Pandorica Opens) The Doctor was released thanks to his companions and his future self and restored the universe with the Pandorica he was initially trapped inside. (DW: The Big Bang) The second and third attempts was to turn Melody Pond into a weapon to kill the Doctor in 1938 Berlin and later 2011 Lake Silencio. Both failed, although the Silence thought that the Doctor had died when he allied himself with the Teselecta to fake his death at Lake Silencio. (DW: The Impossible Astronaut, Let's Kill Hitler, The Wedding of River Song)

Dorium Maldovar told the Doctor that "silence will fall when the question is asked" was a poor translation. A better translation would be "silence must fall...". Although the Teselecta computer, who possessed great knowledge about time, space and the Universe in general, were unable to tell the Doctor what the Question was, Dorium somehow learned it and told the Doctor that the Question was actually "Doctor Who?". (DW: Let's Kill Hitler, The Wedding of River Song)