The clusters interacted gravitationally, causing the matter in both to slow down as they passed through each other. The Bullet Cluster is a cluster of galaxies created by a collision between two other galaxy clusters. This has left a signature in the sky called baryon acoustic oscillations, reminiscent of the ripples of a stone splashing into water.Ĭomparing the predictions of a dark matter theory to observational data from these oscillations, called BAOs, and the overall shape of our universe’s “cosmic web” are necessary tests of the theory’s strength.

When the universe cooled enough that atoms could form, these atoms (“baryons”) were pulled into the dark matter structures, falling into their gravitational potential wells. But dark matter was immune to this interaction and, with the help of gravity, began to form its own structures. Photons could drag on these particles, preventing growth. The hot early universe was in part made up of charged nuclei and electrons. If invisible matter makes up much of the mass of a galaxy, then the faster-than-expected regions can be explained. Finding these abnormal dynamics was one of the first hints that we might be missing something. However, astronomers first noticed in the early 20th century that the opposite was happening: The matter in the outer regions of galaxies was moving much faster than expected.

The prediction was that matter closest to the center of a galaxy would move the fastest, with matter farther from the center feeling less of the pull of gravity and moving more slowly.

Three commonly cited unusual astrophysical observations credited to dark matter are abnormal galaxy rotation speeds, patterns in the large-scale structure of the universe, and the Bullet Cluster.īased on the visible matter in galaxies, physicists can use the laws of gravity to predict how quickly stars revolve around galaxies. Science hypotheses often come from an abnormal observation-something that appears unexpectedly or acts in an unexpected way.

What is dark matter made of