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The Dark Side of the Universe

By Michelle / March 8, 2002


The universe has just presented humanity with yet another blow to our collective ego. It's not enough that the earth isn't the center of the universe, or even that we're just so VERY tiny and insignificant in the big picture. No, now it turns out that we (meaning the earth, the sun, and all the stars and galaxies out there) aren't even made of the stuff that makes up over 90 percent of the universe. And just what the vast majority of the universe is made of is a major mystery.

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What I'm talking about is called dark matter. The term has been around for a while, but in the last few years, it has taken on a whole new importance. The story begins with the discovery of dark matter which, at first, was only called "dark" because it was stuff that didn't shine like a star. Forty or 50 years ago, the only way astronomers could study external galaxies was to take pictures of them with giant telescopes like Mount Wilson or Palomar. The lovely, visible-light images showed galaxies full of twinkling, glowing stars. And since stars were the only things we could take pictures of, we automatically assumed that they were what made up the majority of a galaxy's mass. Without a lot more thought, astronomers happily began to estimate the masses of galaxies based on the amount of starlight we could focus through our telescopes into our warm, soft, limited eyes.

The first inklings of a problem with this picture started with Vera Rubin. Rubin, among other astronomers, had set out to measure the rotation rates of galaxies. All spiral galaxies, including our own, rotate around their centers. The stars mix and swirl around a little, but, for the most part, everything orbits in the same direction in a slow, stately dance. Rubin hoped to use her observations to better understand the structure and behavior of galaxies, but she hardly expected any major surprises.

But, at the end of her research, instead of understanding galaxies better, Rubin could scarcely explain how galaxies existed at all. According to her measurements, galaxies were spinning too fast. Not just a little too fast, but well over twice what their spin rates should be, given their calculated masses. So, what's the wrong with a few dizzy galaxies? The problem was that the stars should have obtained escape velocity. In other words, the stars were moving so fast that the gravity of the galaxy shouldn't have been able to hold the stars together. Ever seen a clown spin a plate on a stick? Try giving the clown a plate full of peas and you begin to get the idea. The galaxies were spinning so fast that the stars should have been flying off in all directions.

Since galaxies do manage to stay together, Rubin made the straightforward conclusion that they must have more mass than previously thought. Since this mass must be something other than stars, the search was on to find out just what this "missing mass" or "dark matter" really was.

At first the ideas weren't all that strange; there were plenty of things that would be hard to see in a visible-light telescope. Maybe there were vast clouds of dust and gas drifting between the stars. Some astronomers suggested there might be billions of giant planets like Jupiter roaming undetected around the galaxy. Others thought the mass might be hiding inside black holes. But the persistent thorn in astronomer's sides was the sheer amount of stuff that seemed to be hiding. Rubin's observations suggested that we weren't just missing a little matter, but the majority of the mass of the universe.

As technology got better, we began to use invisible light to search for the missing mass. Lots of things don't give off any visible light (including all those things mentioned above), but radiate quite happily in other wavelengths of light. And yes, when we turned our new instruments to the sky, we found lots of stuff we'd overlooked before. Infrared telescopes took pictures of hundreds of giant rogue planets, unassociated with any star or solar system, meandering around the galaxy. Vast clouds of cold dust and gas, in some cases stretching hundreds of thousands of light years farther than the "edge" of galaxies (judging from where the stars stop) have been imaged with radio waves. Even black holes, which by definition don't give off any light, can be sniffed out with sensitive X-ray space telescopes like Chandra.