Does Dark Matter Exist, Or Is Gravity Wrong? The Answer Lies Billions Of Years In The Past

When we look out at the luminous matter in the Universe -- stars, galaxies, clusters of galaxies and the hot gas in and between them -- it tells a couple of different stories. One is the story of how normal matter (based on atomic nuclei and electrons) comes together to emit, absorb and otherwise interact with light: an indispensable part of how we see the Universe. But another story is that of gravitation. By observing how this matter moves relative to its surrounding environment, we can learn an awful lot about the gravitational interaction in the Universe. One of the greatest surprises that the 20th century had in store for astronomers was that if you look at the gravitational effects of these large structures, normal matter alone isn't enough to account for it.

If you measure the individual velocities of galaxies within a large galaxy cluster, such as the Coma Cluster (above), you can infer how much mass must be present in order to keep the cluster from flying apart. That number is not only about 50 times greater thanthe amount of stars that are present, but approximately a factor of six larger than all the stars, planets, gas, dust, plasma, and all other forms of normal matter combined. There would seem to be but two simple options that exist as solutions to this: either there's a new, unseen form of mass that's present, dark matter, or the laws of gravity on the largest scales depart from the predictions of Einstein's General Relativity, some form of modified gravity.