Astronomers using data from NASA’s Hubble Telescope have observed what appears to be
a clump of the dark matter left behind from a wreck between massive clusters of galaxies. The result could challenge current theories about the dark matter that predict galaxies should be anchored to the invisible substance even during the shock of a collision. Abell 520 is a gigantic merger of galaxy clusters located 2.4 billion light-years away. Dark matter is not visible, although its presence and distribution is found indirectly through its effects. Dark matter can act like a magnifying glass, bending and distorting light from galaxies and clusters behind it. Astronomers can use this effect, called gravitational lensing, to infer the presence of dark matter in massive galaxy clusters. This technique revealed the dark matter in Abell 520 had collected into a “dark core,” containing far fewer galaxies than would be expected if the dark matter and galaxies were anchored together. Most of the galaxies apparently have sailed far away from the collision. One way to study the overall properties of dark matter is by analyzing collisions between galaxy clusters, the largest structures in the universe. Studies of Abell 520 showed that dark matter’s behavior may not be so simple. Using the original observations, astronomers found the system’s core was rich in dark matter and hot gas, but contained no luminous galaxies, which normally would be seen in the same location as the dark matter. NASA’s Chandra X-ray Observatory was used to detect the hot gas. The astronomers then turned to the Hubble’s Wide Field Planetary Camera 2. The team proposed numerous explanations for the findings, but each is unsettling for astronomers. In one scenario, which would have staggering implications, some dark matter may be what astronomers call “sticky.” Like two snowballs smashing together, normal matter slams together during a collision and slows down. However, the dark matter blobs are thought to pass through each other during an encounter without slowing down. This scenario proposes that some dark matter interacts with itself and stays behind during an encounter. Another possible explanation for the discrepancy is that Abell 520 has resulted from a more complicated interaction than the Bullet Cluster encounter. Abell 520 may have formed from a collision between three galaxy clusters, instead of just two colliding systems in the case of the Bullet Cluster. A third possibility is that the core contained many galaxies, but they were too dim to be seen, even by Hubble.