We Have No Idea: A Guide to the Unknown Universe

by Jorge Cham

As the two galaxy clusters slammed into each other, the gas and dust from the two clusters collided with spectacular results: big explosions, giant clouds of dust getting ripped apart. It’s a special-effects extravaganza. If it helps, visualize the collision of two huge piles of water balloons tossed at each other at crazy high speed. But astronomers also noticed something else. Close to the collision site, they noticed two giant clusters of dark matter; of course, this dark matter was invisible, but they could spot it indirectly by measuring the distortion the clusters were causing to the light from the galaxies behind them. These two dark matter clusters seemed to be moving along the line of collision as if nothing had happened. What astronomers have pieced together is this: there were two galaxy clusters, each with both regular matter (mostly gas and dust with some stars) and dark matter. When the two clusters collided, most of the gas and dust crashed together in the way you expect normal matter to do. But what happens when dark matter bumps into other dark matter? Nothing that we could detect! The clusters of dark matter kept going and passed through each other—almost as if they were invisible to each other. The stars also mostly passed through, because they were so sparse. Enormous blobs of matter, bigger than many galaxies, passed right through each other. In essence, the collision stripped the gas and dust from these galaxies.