Scientists studying early galaxies were stunned earlier this year when they discovered six massive galaxies that seem to have died during the universe's most active period of star birth. NASA's Hubble Space Telescope spied the six galaxies, which appeared to have run out of the cold hydrogen gas needed to make stars while most other galaxies were producing new stars at a rapid pace.

"At this point in our universe, all galaxies should be forming lots of stars. It's the peak epoch of star formation," Kate Whitaker, a University of Massachusetts, Amherst assistant professor of astronomy and lead author of a recent study on the six galaxies, said in a press release. "So what happened to all the cold gas in these galaxies so early on?"

Without the cold hydrogen gas necessary to fuel stars and birth new ones, the galaxies are essentially dead. They're also unable to rejuvenate, even if they have absorbed nearby smaller galaxies and gas clouds. Whitaker said that the act of absorbing just "puffs up" the dead galaxies.