The Silence

The Silence describes an unknown but powerful external force or group 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.

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

In 1580 Venice, Rosanna Calvierri said that through some of the 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 to that place and time, before it snapped shut behind them. 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 existance. (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 repeats the line "Silence will fall / Silence will fall all around". (DW: A Christmas Carol)

Behind the scenes

 * Steven Moffat has confirmed in Doctor Who Confidential that the Silence will be featured in Series 6, with its identity possibly revealed. 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. It was later confirmed Piers Wenger in Sci Fi magazine, in reference to series 6, that the Silence is the name of a terrifying species known as Silents who posses both physical and non-physical forms.
 * The actor who portrayed the voice of "The Silence" is as of yet unknown, but rumoured to be Steven Moffat himself due to the similar sounding voice. Or it could be regular monster voice Nicholas Briggs doing an impression of Moffat.