No Extra Gravity for Dark Matter
The Milky Way is gradually pulling apart a smaller orbiting neighbor known as the Sagittarius dwarf spheroidal galaxy. Just as gravity from the moon causes Earth's oceans to bulge, so too does the gravity of the Milky Way create enormous tides that deform Sagittarius. These are so strong that they rip stars out of the galaxy, producing two long streams of stars, one stretching ahead of Sagittarius and one lagging behind. By observing the stars streaming out in both directions, the researchers conclude that the dark matter and ordinary matter within the smaller galaxy feel the same pull from the Milky Way.
Update (10/11/06): This observation is quite analogous to the (probably apocryphal) story of Galileo dropping objects of different materials and densities from the tower of Pisa. Since the objects take the same time (neglecting air resistance) to fall, this shows that gravity exerts the same force regardless of the type of matter it is acting on.
How Fast Does Dark Matter Fall?
Tags: dark matter, cosmology, gravity