The Silence

The Silence  was a mysterious species of tall humanoids with grey, skeletal-like fleshy faces that forced the Doctor's TARDIS to explode on the date of June 26th 2010, and thus created cracks in time that would erase various things throughout the universe, and eventually the universe itself.

They were apparently present on Earth for hundreds of years, going unnoticed by the human race. If anyone saw one of them properly, they would instantly forget as soon as they looked away. They could use an electrical discharge from their hands to kill someone.

One of them visited the White House and killed Joy. (DW: The Impossible Astronaut)

Prisoner Zero told the Eleventh Doctor that "silence will fall". (DW: The Eleventh Hour)

In 1580 Venice, Rosanna Calvierri said that through some of the cracks, the Saturnynians saw only silence and the end of all things, implying that the silence came from the cracks in time. She would later go on to mention that her race "ran from the silence" in fear, through a crack that acted as a portal. Following his defeat of Rosanna, when the Doctor was preparing to leave, a mysterious silence fell over Venice. The Doctor asked Rory Williams "can you hear that?" and Rory replied "Hear what? All I can hear is silence". There was then a shot of a previously busy Venitian market showing that everybody had mysteriously vanished as if they had been removed from time, space or possibly existence. (DW: The Vampires of Venice)

When the Silence first took control of the TARDIS, they forced it to land outside of Amy Pond's house on June 26th 2010, and began to explode shortly afterwards, trapping River Song inside of it within a time loop until the Doctor saved her. Although the entity behind the voice was never visible or named, it was heard to be speaking before the TARDIS exploded, saying: "Silence will fall!" (DW: The Pandorica Opens/The Big Bang).

The final lines of Abigail's Song repeated the line "Silence will fall / Silence will fall all around". (DW: A Christmas Carol)

Behind the scenes

 * Moffat has implied that the Silence is a group of people several times. On Doctor Who Confidential, he said "who are the Silence?", and in an interview said that Series 6 was about who "they" are, implying it to be a group or number of people and not simply one.
 * The actor who portrayed the voice of "The Silence" is as of yet unknown.