What is dark matter?

Susanna Pilny for redOrbit.com – Your Universe Online

About 1/4 of the universe is composed of dark matter, so it’s kind of a big deal.

Or, to put it another way, up until about 30 years ago, most scientists believed that the universe was composed of atoms—the tiny pieces of each chemical element—and nothing else. Now, most suspect (but haven’t yet proven) that the universe is about 4.6% atoms, 71.4% dark energy (which is a topic for another time), and 24% dark matter.

Fritz Zwicky was the first one to realize that something was wrong. In science, it’s well known that anything that has mass has its own gravity, and that gravity increases as the mass of an object increases. But in some parts of the universe, the masses of objects don’t match the effects of gravity that are seen.

Zwicky was looking at a cluster of galaxies known as the Coma Cluster when he realized that the relationship between mass and gravity wasn’t working as planned. Previously, he had determined the mass of the cluster by examining the amount of light given off in pictures, a common technique still used today.

But he also used an equation from traditional physics—Newton’s law of universal gravitation—which is used when looking at bodies of matter in space that have objects orbiting around them (satellites). The equation can be used to determine the relationship of the distances between the central body and its satellite, and how fast the satellite needs to move to stay in orbit.

Using this equation, the mass came out significantly larger than with the picture. Zwicky theorized that something else—something that didn’t emit light—was adding the extra mass and thus increasing the force of gravity in the cluster. He named it dark matter.

Zwicky’s ideas weren’t granted much traction for a while. Einstein’s theory of relativity helped to explain several places where Newton’s theory failed, but even Einstein’s theory didn’t work in some instances, leading scientists back to Zwicky and his dark matter. Dark matter—an unseeable, currently unmeasurable, weakly interacting substance that affects gravitational relationships between objects in space—can account for these discrepancies.

Dark matter matters

There are several theories as to what dark matter could be—one of the newest is that it is made of decayed Higgs bosons, the particles that are responsible for giving mass to matter. The stuff is still very mysterious, though. It was recently discovered that dark matter doesn’t experience friction when collides with other dark matter, and that the amount of dark matter relates to how large the black holes in the center of galaxies are.

However, there is another solution that has nothing to do with dark matter: like Newton’s equation, Einstein’s could only work part of the time because it is incomplete. And because dark matter hasn’t yet been found, we have no way to know for certain which is right.

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