Video Short: The Dark Matter Mystery
See how a cosmic collision provides evidence that dark matter really exists.
By Melissa Salpietra
We can't see dark matter, and some skeptics doubt its existence, but many scientists think it makes up more than 20 percent of our universe. Astronomer Doug Clowe explains how the Bullet Cluster, a group of galaxies billions of light-years away, may shed some light on this mysterious stuff.