Cosmology 101: Dark matter

Astronomy magazine Associate Editor Liz Kruesi gives an overview of dark matter, that mysterious stuff that makes up some 90 percent of the universe's mass.
By | Published: August 18, 2009
In this series I give you an overview of important ideas in the area of cosmology. This video is the second in the series, and focuses on dark matter.

Using various detectors and research methods, astronomers have determined that the stuff we see in space — stars, gas, and dust — amounts to less than 5 percent of the universe. This stuff is ordinary matter, and it’s made up of protons, neutrons, and electrons. Scientists call ordinary matter “baryonic matter” because protons and neutrons are subatomic particles called baryons.

So if ordinary matter is only about 5 percent of the universe, what’s the other 95 percent? Well, about 23 percent is something called dark matter. The remaining 72 percent is a mysterious pervasive force called dark energy. I’ll explain more about dark energy in my next video.

Alright, back to dark matter. This mysterious mass is a type of matter that doesn’t emit, absorb, or reflect any type of light (so, for example, it doesn’t emit X rays or absorb infrared radiation). Dark matter is therefore invisible. If it’s invisible, how do astronomers have any idea it’s there? Dark matter interacts with ordinary matter through gravity. Its gravitational interaction is how astronomers first found out dark matter exists.

In 1933, a Swiss astrophysicist by the name of Fritz Zwicky first proposed dark matter’s existence. While studying the Coma galaxy cluster, he found that the galaxies’ gravity alone was much too small to hold the cluster together.

The next round of evidence came about 40 years later, in the 1970s. Astronomers charted the velocity of stars at various distances from the center of a galaxy, and plotted the velocity versus the distance. This plot is called a rotation curve. They expected the velocities to reach a maximum and then decrease farther from the center — but the data showed otherwise. The velocities reach a maximum and then plateau. With velocities of this magnitude at the outer edge of galaxies, the stars should be flung out of their orbits. But they aren’t. So some sort of mass that we can’t detect must hold these outer stars in orbit.

Massive galaxy clusters also show evidence of dark matter. A very massive object — such as a galaxy cluster — can bend and magnify light from galaxies behind it. That massive object acts as a gravitational lens. In this image of galaxy cluster Abell 2218, you’ll see numerous blue arcs. Those are background galaxies distorted and magnified by the cluster’s gravity. Astronomers study the sizes and shapes of those arcs to determine the cluster’s mass. By comparing that calculated mass to the mass that comes from only luminous objects (the galaxies), they can determine how much dark matter is in the cluster.

Today, astronomers are pretty confident that dark matter does in fact exist. They’ve created numerous “dark matter maps” using different techniques, including gravitational lensing of galaxy clusters. The next step is finding out what it is. As I said earlier, dark matter doesn’t emit, absorb, or reflect any type of light. So, it’s likely some sort of mysterious non-baryonic particle, meaning it’s not made up of the same stuff as ordinary matter (protons and neutrons).

Astronomers split non-baryonic dark matter into two categories: hot and cold. These titles don’t mean that if you touch them, you’ll feel something that’s hot or cold. Hot means that early in the universe, these particles traveled very, very fast — almost at the speed of light. Cold means that early in the universe the particles traveled slower.

How does particle speed relate to figuring out what dark matter is? Well, slower particles will bunch up into small structures earlier in the universe. Those small structures will eventually collide and merge, forming larger structures. Astronomers believe this is how structure forms and evolves in our universe — smaller structures eventually merge into the massive superclusters and filaments we observe today. Astronomers simulate structure evolution with cold dark matter, and create models that resemble today’s universe. This simulation shows dark matter distribution. Brighter areas represent more dense regions.

OK, so what is cold dark matter? That’s a good question. Scientists aren’t sure yet, but they believe it’s a massive, charge-neutral as-of-now hypothetical particle called the neutralino. They haven’t found this particle yet, but the Large Hadron Collider particle accelerator may change that after it restarts in the fall of 2009.

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