By Paul Sutter, Astrophysicist | August 25, 2017 07:04am ET

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Dark matter is more than a name ? it's a part of our universe. But it's totally unfamiliar to our everyday experience. Based on the evidence, scientists think it's invisible in the truest sense of the word: It simply doesn't interact with light. Gravity, however, is universal, and so dark matter can still have an influence on the shape and motions of galaxies. But we'll never see it. At least, not directly.

As much as we would prefer to live in a simpler universe, dark matter is not the product of some astronomer's fever dream after a late-night observing session. It's only after decades of careful observations that cosmologists have come to the inescapable conclusion that most of the matter in our universe is simply invisible. [Dark Matter and Dark Energy: The Mystery Explained (Infographic)]

Too hot

The initial hints of dark matter came in the 1930s as astronomer Fritz Zwicky made the first X-ray observations of the Coma Cluster, a dense knot of a thousand galaxies over 300 million light-years away. The galaxies themselves aren't very bright in X-ray light, but the galaxies in a cluster swim in a hot, thin soup of plasma (a gas with some unique properties), which does emit high-energy radiation. In his initial measurements, Zwicky noticed an inconsistency: The plasma was much too hot.

Stable systems like galaxy clusters are a study in balance. In this case, the tendency of a hot gas to expand is balanced by the inward pull of its own gravity. If clusters are to survive for billions of years ? which they must, in order for us to actually observe them all over the universe ? then these two forces must be in equilibrium. But when Zwicky added up the masses of all the galaxies and the plasma itself, it was far too small; the inward gravitational pull of all that matter wasn't enough to overcome the natural expansion of the gas. In other words, the cluster should've ? well, I don't want to say exploded, but you get the idea, long ago.

More:
https://www.space.com/37937-evidence...e=notification