Even smaller galaxies hint of dark matter

New Scientist : Astrophysicists had thought that only the collision of massive galaxy clusters, such as the famous Bullet cluster , would show evidence of dark matter, but now researchers using the Canada-France-Hawaii Telescope on Hawaii’s Mauna Kea have found much smaller collisions that also indicate the presence of dark matter. When two galaxies collide, the stars pass by each other. However, electromagnetic forces between the atoms of the interstellar medium create drag, causing them to trail behind in a cloud of gas. Dark matter is not affected the same way and tends to stick with the galaxies’ stars. Using the clusters as a gravitational lens, the researchers were able to estimate their dark matter content by gauging how much their gravity distorts the images of the galaxies behind them. Because small collisions are about 1000 times as common as large ones, astronomers hope to find many more examples, from which they should be able to gather more data on dark matter.