Based on the slightly warped shapes of galaxies, mass—mostly dark matter—in each cluster is mapped and shown in blue.
In visible light, the relationship between two massive galaxy clusters is unclear.
X-rays show that the clusters collided, with the cooler, stripped core of the smaller cluster plowing through the warmer gas.
Dark matter is moving ahead of the gas because it does not interact with normal matter, so it did not collide and slow down.
A Story Of Dark Matter: Bullet Cluster
Multiple wavelengths shed light on the dark universe.
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
Also known as:
Distance from Earth:
3.8 billion light-years
Location in the sky:
Did you know:
At the time these observations of the Bullet Cluster were made in 2004, the collision of the two galaxy clusters was the most energetic event known to have occurred since the big bang set off the expansion of the universe.
Explore More About Dark Matter
Find out more with these additional resources from NASA’s Universe of Learning
Credits: Bullet Cluster
Visible light image from the Hubble Space Telescope, NASA, ESA and the Magellan Telescope, University of Arizona
Dark matter map: NASA, STScI; ESO; University Arizona
X-ray light image from the Chandra X-ray Observatory: NASA, CXC
Composite image (visible, dark matter map, X-ray): NASA, ESA, ESO, University Arizona
Subject-matter expertise provided by Dr. Maruša Bradač
Produced by the Space Telescope Science Institute’s Office of Public Outreach