The Silence

They've been here since fire and the wheel. But they're not human. No, these are aliens I'm talking about. Called simply the Silence, these creatures only remain in your mind while you're looking at them. As soon as you turn your back or even look away, they erase themselves from your memory, as well as, over time, electronic recordings. So don't be surprised if this post randomly vanishes.

Appearing as tall humanoids with bulbous heads, their eyes are set deep in their head, giving them a skull-like appearance. They act as parasites, having other beings build things for them through something similar to post-hypnotic suggestion. It was the Silence that took over the TARDIS, forcing it to explode throughout all of time and space on June 26, 2010, causing the end of the universe. The Doctor was able to reboot the universe, and was saved by Amelia Pond's memories, but that's another story (The Pandorica Opens/The Big Bang).

It was the Silence who arranged the moon landing in 1969, for what reason is not clear. The Doctor arrived at their secret base, where they were holding Amy Pond, and sets up a television in time to see the "One small step" speech. He hacked into the signal just as Neil Armstrong's foot touched the surface surface of the moon. The image turned to a video shot by Canton Everett Delaware III of the Silence saying 'you should kill us all on sight". By doing so, the Silence have planted the post-hypnotic suggestion in everyone on Earth, as it is the most famous television broadcast in history. 

"You should kill us all on sight."

But there are still unanswered questions about the Silence.
Why did they force the TARDIS to explode? Why did they put the cracks in the fabric of the universe? Why did they imprison the little girl in the space suit, and who is she? We can only wait and see.
But wait--what's that in the corner of your eye? What are those tally marks covering your arms?