The Silence

The Silence  was a mysterious species 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.

Biology
The Silence are depicted as tall humanoids with bulbous heads and grey, mouthless, skeletal-like fleshy faces. They speak in low, guttural voices. (DW: The Impossible Astronaut)

Powers and abilities
If anyone saw one of them properly, they would instantly forget as soon as they looked away. The Silence could also use an energy discharge from their hands to kill someone. (DW: The Impossible Astronaut)

History
The Silence first took control of the TARDIS,in 102 A.D.. They forced it to land outside of Amy Pond's house on June 26th 2010, and began to explode shortly afterward, 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).

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 Earth. (DW: The Vampires of Venice)

The Silence were apparently present on Earth for hundreds of years, going unnoticed by the human race. One of them spied on the Doctor, Amy, Rory, and River in Utah, 2011. Another one visited the White House in 1969 and killed Joy in front of Amy Pond. (DW: The Impossible Astronaut)

Behind the scenes

 * Marnix Van Den Broeke portrayed the Silence in The Impossible Astronaut and Day of the Moon