Similar physics mystery: You're a pool of water in the bottom of a bucket, looking out at the stars. Somebody spins the bucket, making the stars spin. So you (the water) arrange your molecules in a parabolic shape, thicker at the sides of the bucket and thin in the middle.

How do you (the water know)? How do you know that the stars aren't holes in a paper sheet with light shining through? How do you know the universe isn't spinning, and you're standing still?