NOVA scienceNOW | The Dark Matter Mystery | PBS

http://www.pbs.org/nova/sciencenow We can't see dark matter, and some skeptics doubt its existence, but many scientists think it makes up 20-some 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. Don't miss the new season of NOVA scienceNOW, airing every Wednesday at 9pm starting June 25 on PBS. Watch past episodes of the program, try out interactives, and more on our Web site: http://www.pbs.org/nova/sciencenow