Dark Matters

Under dark skies, two galaxies are in good view tonight: Andromeda and our own galaxy, the Milky Way. As night falls, the Milky Way forms a faintly glowing band across the northern sky, arcing from Orion in the east to the Summer Triangle in the West. And Andromeda stands just above the apex of the Milky Way’s arch. It looks like a small, hazy patch of light.
These and many other galaxies formed the basis of one of the most important discoveries of the 20th century: dark matter. It emits no detectable energy, but its gravity pulls at the visible matter around it.
Astronomers discovered dark matter in part by measuring the motions of stars. If galaxies contained only stars, gas, and dust, then the stars at their edges should move slower than the stars near the center. But observations showed that the stars were all moving at about the same speed, suggesting that something else was accelerating the ones at a galaxy’s rim: dark matter.
A recent study, though, suggests otherwise. The study