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What Is Dark Matter?

Dark matter is one of the greatest mysteries of modern astrophysics and it's been in the news a lot lately. Everyday Einstein explains what it is and why it's been so elusive to find.

By
Sabrina Stierwalt, PhD,
Episode #134

Dark matter filaments

Hi I’m Dr. Sabrina Stierwalt, the Everyday Einstein, bringing you Quick and Dirty Tips to help you make sense of science.

What if I told you that the form of matter that you and I know well - the matter that makes up us, our homes, our pets, our Earth – accounts for less than 5% of the stuff in the universe? That, in fact, another mysterious form of matter dominates and, despite decades of searching for it, not only have astrophysicists never detected it, but we don’t even know what it is.

Would you believe me?

Dark matter is one of the greatest mysteries of modern astrophysics, but you don’t have to take my word for it. Not only is this elusive mystery matter a fascinating topic, but the search for dark matter is also a great example of how scientists approach unsolved problems. We are living at a great time for dark matter studies that appear right on the cusp of finally making a direct detection.

In this episode, we’ll explore how we know dark matter exists and why it has been making the news lately.

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Dark Matter Defined

Astrophysicists know that dark matter must be everywhere, but we also don’t yet know its make up. It doesn’t interact with the “normal” matter that we understand and emit radiation, so it can’t be observed by telescopes. Thus we call it “dark.”

We have determined the universe to be 26.8% dark matter, 4.9% normal matter (the stuff you and I are made of), and 68.3% something called dark energy. This breakdown is based on data collected over the whole sky from telescopes like the Wilkinson Microwave Anisotropy Probe and more recently Planck.

How Do We Know Dark Matter Exists?

As much as we are unsure what makes up dark matter, we are still very sure that it exists.

How can this be? Well, even though we can’t detect dark matter directly, there is a lot of indirect evidence as to its existence.

First, let’s consider what it means to have evidence that is indirect. If you were to visit my house, you would suspect that a toddler lives there even if you didn’t see her. You’d see toys littering the floor and sippy cups out on the counter. Of course, this evidence is circumstantial – maybe I enjoy hammering with plastic hammers and having drinks on the go?

Evidence that would be harder to explain without a toddler, however, might be the tiny finger smudges all over the windows and television. You still don’t have direct evidence of her existence without a confirmed toddler sighting, but those fingerprints would be hard to produce any other way.

In the 1970s, astronomer Vera Rubin was the first to make measurements offering indirect evidence of the existence of dark matter while at the same time linking those measurements to dark matter as the cause. Her observations tracked the speeds of stars orbiting within spiral galaxies.

As we discussed in a previous episode, bodies in orbit are moving at just the right “Goldilocks” speed. For example, if the Moon were to somehow slow down in its orbit around the Earth, the Earth’s gravitational pull would take over causing the Moon to fall inward. If the Moon instead were to speed up, the force associated with its motion would win out over Earth’s gravity, and the Moon would fly off into space.

See also: Can We Feel the Earth Moving?

 

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About the Author

Sabrina Stierwalt, PhD

Dr. Sabrina Stierwalt is an extragalactic astrophysicist at the California Institute of Technology and Adjunct Faculty at the University of Virginia.

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