The Matter of the Bullet Cluster

Explanation: What's the matter with the Bullet Cluster? This massive cluster of galaxies ( 1E 0657-558 ) creates gravitational lens distortions of background galaxies in a way that has been interpreted as strong evidence for the leading theory: that dark matter exists within. Different recent analyses , though, indicate that a less popular alternative -- modifying gravity-- could explain cluster dynamics without dark matter , and provide a more likely progenitor scenario as well. Currently, the two scientific hypotheses are competing to explain the observations: it's invisible matter versus amended gravity. The duel is dramatic as a clear Bullet-proof example of dark matter would shatter the simplicity of modified gravity theories . For the near future, the battle over the Bullet cluster is likely to continue as new observations , computer simulations, and analyses are completed. The featured image is a Hubble / Chandra / Magellan composite with red depicting the X-rays emitted by hot gas, and blue depicting the suggested separated dark matter distribution.