By Ron Cowen
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.
![](https://i0.wp.com/www.sciencenews.org/wp-content/uploads/2007/08/7039.jpg?resize=300%2C287&ssl=1)
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.