The speeds of stars on circular orbits have been measured around both spiral and elliptical galaxies. Credit: M. Cappellari and the SLUGGS team
The speeds of stars on circular orbits have been measured around both spiral and elliptical galaxies. Without dark matter, the speeds should decrease with distance from the galaxy, at different rates for the two galaxy types. Instead, the dark matter appears to conspire to keep the speeds steady. Credit: M. Cappellari and the SLUGGS team