A Story Of Dark Matter: Bullet Cluster

Dark matter is an enigma; scientists know more about what it is not than what it is. The mystery makes it one of the most exciting areas of astronomy. Though dark matter has not been detected with telescopes, we know it exists because of its effect on objects we do see—objects that emit or reflect light. When space is warped by dark matter’s gravity, the light of distant galaxies appears distorted. Using this and other methods, astronomers calculate that there is much more undetectable dark matter in the universe than detectable, “normal” matter.

The Bullet Cluster is composed of two clusters of galaxies that collided and moved past each other, though this is not clear when viewing the region solely in visible light. Multi-wavelength observations of the Bullet Cluster provided the first strong observational evidence that dark matter does not interact with normal matter, or with itself, and holds the majority of mass in a galaxy cluster. Astronomers use visible-light images to map the location of the clusters’ mass, based on how the light of background galaxies is warped. Most of that mass is dark matter. X-rays show that the majority of normal matter, in this case gas, is in a different location than the dark matter of each cluster—it lags behind. This is because the normal matter of the two galaxy clusters collided, while the dark matter sailed through and kept going without interacting at all. Many mysteries remain as to the nature of dark matter, and the Bullet Cluster provides key evidence in the scientific investigation.

Quick Facts: Bullet Cluster