[ well aint that a question for the ages. He's not actually sure he's ever had to explain that before. ]

Uh, celestial beings, probably aliens. Cas said once he was the size of the Chrysler building in his true form, but I've never seen it. He said it'd probably kill me. So I'm thinking they can only reveal the goods to each other, in another dimension or something. I've only ever seen the shadow of his wings.