To measure the dark matter, astronomers used Hubble's Advanced Camera for Surveys to snap pictures of the galaxy cluster Abell 1689, located about 2.2 billion light-years away. The cluster is so massive that its gravity warps the light from galaxies behind it, an effect called gravitational lensing. Faraway galaxies appear as warped smears or distorted blobs, like you would see in a funhouse mirror. By studying the warped smears, astronomers determined how much mass is in the Abell cluster. (More mass leads to more distortion.) Then they could subtract that from the mass they could see — stars and the like — to figure out the amount of dark matter, and its distribution throughout the cluster.