Silents

The Silence is a malevolent alien race from Doctor Who, who have been labelled as one of the scariest monsters in the show, rivalling the Weeping Angels: though this is not without controversy as some have noted a resemblence to the famous Buffy monsters, the Gentlemen - as well as familiar psychological tricks that have become a staple of modern Doctor Who.

In essence The Silence embody the fear of the unknown (or more accurately the unknowable) and are inspired by the Men in Black of UFO conspiracy, faceless monstrosities they manipulate the course of human evolution but erase the memories of any who see them (specifically the moment they look away): making them invisible to all but the most clever of minds.

In order to combat the Silence members of the Doctor's team mark themselves each time they see one of these creatures, so as to combat the loss of memory they induce - after a confrontation with these beings the Doctor managed to implant a command in humanity that subliminally tells them to kill the Silence on sight: making their plans for conquest a lot more difficult.

The Doctor notes that the Silence may of had humanity land on the moon for the purpose of creating what is now known as the famous space-suit - which they no doubt planned to utilize in latter plans.