The Silence

The Voice describes an unknown, but powerful entity that forced the Doctor's TARDIS to explode, and thus created the Cracks that would erase the Universe. (DW: The Pandorica Opens/The Big Bang)

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)

Behind the Scenes

 * Steven Moffat has confirmed in Doctor Who Confidential that the Voice will be featured in Series 6, with its identity possibly revealed.
 * As its actions nearly caused the whole of reality to be erased, it is likely the Voice comes from a completely different realm.
 * Going with the above, an entity refered to as "the silence" was seen through the crack, refered to as a who or what in Confidential, it is most likely that, from what the voice did and said, it might be "the Silence" entity.
 * It is rumoured that the Voice is in fact Omega.