Coma Cluster

The Coma galaxy cluster is one of the biggest and brightest around. It contains thousands of galaxies, including quite a few that are far more impressive than our own galaxy, the Milky Way. Even so, most of what makes up the cluster is invisible.
The cluster is in the constellation Coma Berenices, which is in the east at nightfall. It’s well above brilliant Arcturus, which is low above the horizon. You need a telescope, though, to see the galaxies.
So far, no telescope can show us the bulk of the cluster’s mass. That’s because it consists of dark matter. This form of matter produces no detectable energy, but reveals itself through its gravitational pull on the visible matter around it. In fact, dark matter was discovered through its influence on the Coma galaxies.
In 1933, astronomer Fritz Zwicky measured the motions of the cluster’s galaxies. He found that all the visible galaxies couldn’t account for those motions — something else was tugging at them. He called it “dark matter” — but