Galaxies like our Milky Way are famous for their sparkling spiral arms, dotted with diamond-like stars and full of glowing gas and dusty tendrils. They may look like sprays of stars that sweep around the galaxy like a pinwheel, but they’re more like galactic traffic jams—pressure waves that stars and gas pass through as they whirl around the galaxy.

A new study suggests they may have a ghostly shadow—trailing dark matter spirals hovering above and below them. A team of scientists found them in galaxy simulations by looking for traces of a gravitational wake left by the visible spiral arms.