Bullet cluster The Bullet cluster (1E 0657-558) consists of two colliding clusters of galaxies. Strictly speaking, the name Bullet cluster refers to the smaller subcluster, moving away from the larger one. It is at a co-moving radial distance of 1.141 x 109 parsecs (3.7 billion light-years).[1] Gravitational lensing studies of the Bullet cluster are claimed to provide the best evidence to date for the existence of dark matter.[2][3] However, this interpretation of the gravitational lensing results is disputed. Observations of other galaxy cluster collisions, such as MACS J0025.4-1222, are similarly claimed to support the existence of dark matter. Mordehai Milgrom, the original proposer of MOND (Modified Newtonian Dynamics), has posted on-line a rebuttal of claims that the Bullet Cluster proves the existence of dark matter.[4] Milgrom claims that MOND correctly accounts for the dynamics of galaxies outside of galaxy clusters, and even in clusters such as the Bullet Cluster it removes the need for most dark matter, leaving only a factor of two which Milgrom expects to be simply unseen ordinary matter (non-luminous baryonic matter) rather than cold dark matter. The Bullet Cluster (1E 0657-56) consists of two colliding clusters of galaxies. Strictly speaking, the name Bullet Cluster refers to the smaller subcluster, moving away from the larger one. It is at a comoving radial distance of 1.141 Gpc (3.72 billion light-years). The first known reference to this cluster comes from a 1992 paper titled, "The Einstein Slew Survey". The object is of a particular note for astrophysicists, because gravitational lensing studies of the Bullet Cluster are claimed to provide strong evidence for the existence of dark matter. Observations of other galaxy cluster collisions, such as MACS J0025.4-1222, similarly support the existence of dark matter. The major components of the cluster pair—stars, gas and the putative dark matter—behave differently during collision, allowing them to be studied separately. The stars of the galaxies, observable in visible light, were not greatly affected by the collision, and most passed right through, gravitationally slowed but not otherwise altered. The hot gas of the two colliding components, seen in X-rays, represents most of the baryonic, or "ordinary", matter in the cluster Particularly compelling results were inferred from the Chandra observations of the 'bullet cluster' (1E0657-56; Fig. 2) by Markevitch et al. (2004) and Clowe et al. (2004). Those authors report that the cluster is undergoing a high-velocity (around 4,500 km/s) merger, evident from the spatial distribution of the hot, X-ray-emitting gas, but this gas lags behind the subcluster galaxies. Furthermore, the dark matter clump, revealed by the weak lensing map, is coincident with the collisionless galaxies, but lies ahead of the collisional gas. This—and other similar observations—allow good limits on the cross-section of the self-interaction of dark matter. According to Eric Hayashi: The Bullet Cluster has been claimed as a significant challenge for all theories proposing a modified gravity solution to the missing mass problem, including modified Newtonian dynamics (MOND). Astronomers measured the distribution of stellar and gas mass in the clusters using visible and X-ray light, respectively, and also mapped the gravitational potential using gravitational lensing. As shown in the images on the right, the X-ray gas is in the center, while the galaxies are on the outskirts. During the collision, the X-ray gas interacted and slowed down, remaining in the center, while the galaxies largely passed by one another, as the distances between them were vast. The gravitational potential reveals two large concentrations centered on the galaxies, not on the X-ray gas, where most of the normal matter is located. In ΛCDM one would also expect the clusters to each have a dark matter halo that would pass through each other during the collision (assuming, as is conventional, that dark matter is collisionless). This expectation for the dark matter is a clear explanation for the offset between the peaks of the gravitational potential and the X-ray gas which was detected at a statistical significance of 8σ. It is this offset between the gravitational potential and normal matter that was claimed by Clowe et al. as "A Direct Empirical Proof of the Existence of Dark Matter" arguing that modified gravity theories fail to account for it. However, this study by Clowe et al. made no attempt to analyze the Bullet Cluster using MOND or any other modified gravity theory. Furthermore, in the same year, Angus et al. demonstrated that MOND does indeed reproduce the offset between the gravitational potential and the X-ray gas in this highly non-spherically-symmetric system. In MOND, one would expect the "missing mass" to be centred on regions which experience accelerations lower than a0, which, in the case of the Bullet Cluster, correspond to the areas containing the galaxies, not the X-ray gas. Nevertheless, MOND still fails to fully explain this cluster, as it does with all other galaxy clusters, due to the remaining mass residuals in several core regions of the Bullet Cluster. Mordehai Milgrom, the original proposer of MOND, has posted an online rebuttal of claims that the Bullet Cluster proves the existence of dark matter. He contends that the observed characteristics of the Bullet Cluster could just as well be caused by undetected standard matter. He has argued that all galaxy clusters could host cold dense hydrogen gas clouds of roughly equal to the mass of the visible baryons which could explain the failures of MOND in galaxy clusters. Such cold dense hydrogen clouds are unlikely to exist however due to feedback from AGNs which prevent hydrogen gas from cooling. There are other alternate theories of gravity like the MOG and Many-body gravity (MBG), which claim to be able to explain the bullet cluster's weak gravitational lensing.