The beautiful spinning pinwheel of the Andromeda galaxy, our celestial neighbor, poses a mystery. The breakneck speed of its rotation cannot be explained by applying the known laws of physics to the disk's visible matter. By rights, the gravity generated by the galaxy's apparent mass should cause the stars in the periphery to move more slowly than they actually do. If there were nothing more to the galaxy than the visible matter, then Andromeda—and nearly all such fast-rotating galaxies—simply should not exist.