Michael A. Keim

In the constellation Cetus, 65 million light-years from Earth, three dwarf galaxies nestle: DF2, DF4, and DF9. All have been declared “galaxies without dark matter” — the mysterious invisible substance that usually holds stars together.

A million orbits: the same score. Nine dark matter halos: nine orchestras without a conductor. All musicians play from the same notes, but in each hall the music sounds different: sometimes in tune, sometimes out of sync. Likewise, satellite galaxies falling into a host halo follow orbits that depend not only on mass but also on the shape of the invisible stage.