October 30, 2008---The Bullet Cluster (above) was born of a violent collision between two other large galaxy clusters. It is a mix of dark matter and million-degree gasses that reside 3.8 million light-years from Earth.

Some astornomers theorize that the cluster holds antimatter from the Universe's earliest days.

But the x-ray output in this composite image (red), and a lack of observable gamma rays, suggests that the Bullet Cluster does not contain antimatter in significant amounts, according to NASA.