We don't know what it is, we cannot know. All we can say is that it appears as a unicorn to us. You're just making a special pleading argument from ignorance hybrid. We don't need to give Steven Moffat any more silly ideas.
At the age of five, Skagra decided emphatically that God did not exist. This revelation tends to make most people in the universe who have it react in one of two ways - with relief or with despair. Only Skagra responded to it by thinking, 'Wait a second. That means there's a situation vacant.'
