This article on Bullet cluster is a guest article by Anja Sjöström, an IB diploma student from Switzerland.

The Bullet Cluster is the common name given to the galaxy cluster 1E 0657-56 which was formed by the collision of two clusters of galaxies, pulled together by gravity. This event is considered to be the most energetic one recorded since the big bang. This merger, one of the hottest known in the Universe, is located at about 3.8 billion light years away from Earth and is composed of stars, gas, and a speculated majority of weakly interacting dark matter.

A dynamic merger of two galaxies, seen via Hubble 
Photo Credits: NASA
A dynamic merger of two galaxies, seen via Hubble
Photo Credits: NASA

In most cases, the Bullet Cluster specifically refers to a subpart of the larger cluster which is said to be moving away from the main body at a higher speed due to disparate collision behaviors of the major components of the initial galaxies. In effect, the stars of the galaxies interacted gravitationally during the collision, the gases mostly electromagnetically, causing them to slow down to a greater extent (remaining in the middle of the cluster) and lastly, the mass centroids, found on the outskirts of the cluster, suspected to be composed of non-baryonic dark matter and as such, did not interact to a greater extent than simple gravitational interference.

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Detection and observation of the Bullet Cluster

First and foremost, the stars of the colliding galaxies, which emit in the visible part of the electromagnetic spectrum, were observed with common optical telescopes. The second component of the colliding clusters which account for most of the baryonic matter, the latter were observed at the Chandra X-Ray observatory, due to strong electromagnetic interaction between the hot gases during the collision, these gases emitted strongly in the X-ray part of the electromagnetic spectrum.

X-ray photo of Bullet cluster by Chandra X-ray Observatory.
Image: wikipedia
X-ray photo of Bullet cluster by Chandra X-ray Observatory.
Image: NASA/Chandra

Lastly, scientists were observing objects situated behind the cluster thanks to gravitational lensing. Gravitational lensing is the distortion of light due to the gravitational pull of massive objects, causing the light’s trajectory to be bent as it travels towards the observer here, on or around Earth. As such, the causality was the existence of extremely massive areas within the cluster which would be able to account for a gravitational lens. Putatively, Dark Matter.

Bullet Cluster
This composite image shows the galaxy cluster 1E 0657-56, also known as the “bullet cluster”, formed after the collision of two large clusters of galaxies. Hot gas detected by Chandra is seen as two pink clumps in the image and contains most of the “normal” matter in the two clusters. An optical image from Magellan and the Hubble Space Telescope shows galaxies in orange and white. The blue clumps show where most of the mass in the clusters is found, using a technique known as gravitational lensing. Most of the matter in the clusters (blue) is clearly separate from the normal matter (pink), giving direct evidence that nearly all of the matter in the clusters is dark. This result cannot be explained by modifying the laws of gravity.

Dark Matter and MOND

When classical Newtonian dynamics could no longer account for and explain the cohesiveness of galaxies, when the ordinary matter was observed to not be massive enough to prevent galaxies from dispersing, scientists came to acknowledge that a vast amount space was filled with an indivisible and alternate form of matter which would interact in the gravitational scope.


Dark Matter was proposed as one conceptual and theoretical explanation. This form of matter is said to account for around 85% of the matter in the universe and has now come to be a possible implication of various gravitational effects that cannot be elucidated by the accepted theories, playing a strong role in the structure and evolution of the universe.

The Mystery of Bullet Cluster Of Galaxies 1
Image Credits: G.L.A.S.P

Eventually came discordances between different theories which would explain for unresolved phenomena such as the unexplained gravitational lens of the Bullet Cluster. For instance, contrasting with Dark Matter came Modified Newtonian Dynamics (MOND), and this is where its significance would come into play: a make or break for Dark Matter.

The Bullet Cluster as evidence for Dark Matter

The conundrum that arose from the detectable gravitational lens was to find a theory to complete for what the gravitational force law could not explain alone. In effect, the lensing observed could not have been solely due to the effects of the traditional baryonic matter (the hot gases), following the predictions of theories refuting Dark Matter such as MOND.

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For the areas where the strongest lensing was recorded were found at two different ends of the cluster, beyond where the heavily interacting gas would’ve slowed down. As such, it was deduced that most of the mass of the cluster which caused the lensing was located in the areas where predicted Dark Matter was to be found. This, in turn, worked in favor of the existence of Dark Matter.

Furthermore, this claim is in accord with the said properties of Dark Matter. For instance, it is said to be weakly interacting (only gravitational interaction). As such, this would explain why the Dark Matter was able to bypass the gas during the collision and experience a little slowdown, being eventually found at both ends of the cluster. Finally, hypotheses for Dark Matter based on observations of the Bullet Cluster are not the only evidence of the type. For instance, another galaxy cluster collision, MACS J0025.4-1222 also comes in to support this theory.

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