Some 3 billion years ago, two massive clusters of galaxies collided head on. The debris from this ancient cosmic train wreck, astronomers say, might pose a new puzzle about the invisible material believed to account for most of the mass in the universe.
A variety of evidence indicates that this material, known as dark matter, is about eight times as abundant as ordinary matter and that it resides in vast, invisible halos around star-filled galaxies. Dark matter keeps galaxies and galaxy clusters intact, theorists say.
The leading model for dark matter suggests that it interacts only through gravity and can't be pushed around by the strong, the weak, or electromagnetic