The Silence


 * For the species that identified itself as "The Silence", see Unnamed species (The Impossible Astronaut)

 was a religious order. Their core belief was that "silence will fall when the Question [was] asked", "the Question" being the the oldest question, the very first question of the universe, specifically "Doctor who?" (DW: The Wedding of River Song)

Beyond this core belief was the significant goal of killing the Doctor, whom the Silence felt interfered with their central tenets.

History
The Silence was the mysterious entity that forced the Doctor's TARDIS to explode on 26th June 2010, creating cracks in time that would erase things throughout the universe, and eventually the universe itself. Although the entity that caused the Doctor's TARDIS to explode was never visible or named, a voice was heard speaking before the TARDIS began exploding, stating "Silence will fall!" (DW: The Pandorica Opens / The Big Bang)

The Silence also indirectly played a hand in the destruction of the Saturnyns' homeworld. In 1580 Venice, Rosanna Calvierri said that through some of the cracks, the Saturnynians saw only "silence and the end of all things." This caused them to flee their homeworld to 1580 Earth through a different crack that acted like a wormhole. (DW: The Vampires of Venice)

The Silence, seemingly allied with Madame Kovarian, the Church and Order of the Headless, were also behind the Battle of Demon's Run, as well as the kidnapping and brainwashing of a young River Song. She then made several attempts on the Eleventh Doctor's life. Ultimately, she successfully introduced a poison into his system which killed him, though she broke free of her conditioning and offered him the sum of her regenerative energy. (DW: Let's Kill Hitler).

The Silence, however, managed to track River Song down to the Luna University where they kidnapped her and forced her into an astronaut suit as part of their plan to kill the Doctor. (DW: Closing Time).