Invisible dark matter forms a hazy radial structure in this composite image of a galaxy cluster from Hubble Space Telescope. Although the dark matter itself produces no detectable energy, astronomers can map its presence by measuring how it affects the light of the galaxies beyond it. This image shows the galaxy cluster Cl 0024+17. Dark matter appears to make up about 85 percent of all the matter in the universe, although scientists are still trying to deduce its nature. [NASA/ESA/M.J. Lee (Johns Hopkins)]
If you stand under a dark sky late this evening, away from city lights, you’ll see a handful of bright stars. Vega stands high overhead, with yellow—orange Arcturus in the west and pure orange Antares in the southwest, for example. You’ll also see a few thousand fainter stars, plus the Milky Way — the combined glow of millions of stars.
But mostly you’ll see blackness — the dark space between the stars. And that seems appropriate, since the stars themselves account for little of the material that makes up the universe.
More than 80 percent of all the matter in the universe takes the form of dark matter. It produces no detectable energy, but it reveals its presence by exerting a gravitational tug on the visible matter around it.
So far, no one is sure what dark matter is. It’s probably some type of subatomic particle. Astronomers have found hints to support a couple of models, but no confirmation. More about that tomorrow.
After dark matter, most of the rest of the material in the universe is in the form of gas and dust — individual atoms and molecules scattered between stars and galaxies. This material can produce radio waves, X—rays, and other forms of energy. It can also reveal its presence by reflecting or absorbing starlight.
Finally come the stars and planets. Although they’re the bulk of what we actually see in the night sky, they comprise only about one—tenth of all the matter in the universe — the visible tip of the cosmic iceberg.
Script by Damond Benningfield, Copyright 2013
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