All About Dark Matter: What is This Mysterious Substance?

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Scientists use the term “dark matter” to describe something they have evidence for but can’t see — hence the “dark.” They think it’s dispersed throughout the universe. Don’t look now, but it’s around you, too.

Dark matter isn’t evenly spread. It’s bunched up in some regions and drawn out in others. Its job is simple: holding the universe together.

This cosmological adhesive is likely an entirely new type of matter that isn’t close to being understood. “Baryonic matter” –- consisting of protons, neutrons and electrons –- is the familiar stuff that makes up the known solids, liquids and gases in the universe. But that may be only a small percentage of what actually exists.

The most common theory of dark matter is that it’s “non-baryonic.” Instead, it may be composed of weakly interacting massive particles, known, ironically, as WIMPS.

The mass of each WIMP is impressive: 10 to 100 times that of one proton. The “weak” part comes in because WIMPS may not mingle much with baryonic matter. This lack of interaction is one reason dark matter has been hard to pin down.

Some scientists think that WIMPS could be previously considered theoretical particles. Neutralinos are front-runners, but neutral axions and uncharged photinos are also candidates.

Discovery of Dark Matter

Dark matter came under discussion because parts of the universe seemed to be misbehaving. During the 1930s, Fritz Zwicky, a Swiss astronomer, was measuring orbiting speeds of a cluster of galaxies over 320 million light-years from earth.

The fact that the galaxies were orbiting at all was strange. Based on the amount of mass that could be measured, they shouldn’t have been clustered. The star systems should have moved away from each other long, long ago.

Zwicky theorized that an unknown, unseen type of matter, which he referred to as “dark,” allowed the galaxies to continue orbiting. Two decades later, astronomers Vera Rubin and Kent Ford, working at Washington D.C.’s Carnegie Institution, contributed to this theory.

Rubin and Ford began with the assumption that most of a spiral galaxy’s mass is in its core, since that’s the location of most of the visible matter. That means that a galaxy’s gravitational pull would be strongest at the center, as well.

But their observations didn’t support this. Stars at the outer edges of a galaxy should have been moving more slowly, because the gravitational pull was weaker.

However, stars throughout a spiral galaxy moved at the same speed. To explain this, Rubin concluded that galaxies have about 10 times as much hidden mass as visible mass.

Evidence of Existence

Evidence for dark matter is based on the connection between mass and gravity. When a body in space has sufficient mass, it pulls a less massive object into an orbit.

This is apparent in the earth’s solar system. The planets, their satellites and smaller entities orbit the center of the system’s mass, which is near the sun. The gravitational pull on the outer planets is less because of the distance, so they orbit more slowly.

Scientists estimate the orbit speed of a distant star by observing changes in wavelengths of light as the star moves across earth’s line of site. Since some stars seem to be moving far faster than can be accounted for by the visible matter, something else must be at play.

Alternative Theories

It’s possible that dark matter particles haven’t been seen because scientists haven’t yet developed the proper method. And some researchers suggest that dark matter isn’t made of WIMPS at all.

A newer idea is that dark matter is composed of even heavier particles called Planckian Interacting Dark Matter. They may come about due to very high temperatures during the early days of the universe.

Other scientists want to revisit Newton’s and Einstein’s theories of gravity. Perhaps there are no invisible dark matter particles at all. Maybe gravitational theory needs to be changed to accommodate evidence from far-off galaxies.

Dark Matter’s Contribution

If scientists can uncover the secrets of dark matter, they can better explain the formation of the universe — and also predict where it’s heading. Proof of dark matter could provide more information about how galaxies developed following the Big Bang.

What’s in store for the universe? It’s still expanding, but growth depends upon gravity and mass. The amount of mass includes both visible and dark matter.

Knowing dark matter’s nature will help scientists predict if the universe expansion will continue at its current rate, slow down, or, eventually, stop. The result: a total collapse. And wouldn’t it be nice to know that ahead of time?

About the Author: Megan Nichols loves discussing innovations in technology and science. Her favorite topics are astronomy, the environment, and psychology. If you are interesting in learning about other astronomical wonders, join the discussion on her blog, or follow her on twitter.