Science & Technology

How Do We Know Dark Matter Really Exists?

Ever since the late 1960s when Vera Rubin and Kent Ford discovered that galaxies don't behave the way they should, scientists have been looking for the mysterious substance behind that behavior. That theoretical stuff is called dark matter, and while it's invisible to telescopes, it has mass, which means it can show its might through the force of gravity. Of course, that's all theoretical. Some might even say it's a little too convenient, as if scientists just came up with a magical substance that makes the math work. What makes us so sure that dark matter is even a thing?

Hold Me Closer, Tiny Particle

"People ask this question a lot," said Katie Mack, a theoretical astrophysicist at North Carolina State University who studies dark matter. "You know, maybe dark matter is just a fudge factor or something." But for astrophysicists, dark matter is much more than that.

If you go back to high school physics class, you may remember that the more mass something has, the greater its gravitational pull. If galaxies were only made up of the stuff we can see, there wouldn't be enough gravity to keep them together, much less to keep the stars in the sparse outer edges orbiting just as fast as those in the center. In fact, scientists reckon that normal matter makes up less than five percent of the universe. Dark matter seems to make up a whopping 27 percent. (The rest is a mysterious force called dark energy.)

You can also see its mass in the warping of spacetime itself. According to Einstein's theory of general relativity, matter curves the fabric of spacetime the way a bowling ball curves the fabric of a trampoline. When light travels toward that curve, it doesn't go in a straight line. Instead, it follows the curve, bending around the massive object before continuing on its path. That warp in spacetime turns into a sort of cosmic magnifying glass in a phenomenon known as gravitational lensing. But yet again, the gravitational lensing produced by a galaxy or a galaxy cluster is too great to be explained by the matter we can see. This effect is yet another piece of evidence for the existence of dark matter.

You can even see unexpected patterns in the cosmic microwave background, the light left over from the birth of our universe, that point to the existence of dark matter. "We see patterns in that that show there had to have been something at early times that brought matter together in a way that can't work with just regular matter," Mack told Curiosity.

All Signs Point to Yes

It's not as if scientists have never tried to find an alternative. Instead of dark matter, maybe gravity just doesn't behave the same way everywhere in the universe. Researchers have tried tweaking gravity to make galaxies rotate differently than Einstein's theories say they should, sans dark matter, and it can work — but then other observations we've made don't match up. In the end, Mack says dark matter is the most likely explanation we've got. "I'm not sure what it is that makes it more appealing to break general relativity and say that all these huge observations are wrong, versus 'there's a particle we can't see."

"It's kind of like if you were walking down the street and you see a plastic bag sort of move across the street in front of you," she said. "And then you see some trees lean over, and then you hear this kind of rustling sound, and then you feel a little bit of cold coming from one direction, and then you see a street sign swing, and you're like 'That's wind!' You can't see the wind, but there are all of these different pieces of evidence that air is moving.

"It's kind of the same with dark matter. We can't see it directly, but there are so many pieces of evidence that it just makes more sense than any other explanation that we can come up with."

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Katie Mack is currently writing a book of her own, but while you wait, check out "The 4 Percent Universe: Dark Matter, Dark Energy, and the Race to Discover the Rest of Reality" by Richard Panek to learn more about the search for dark matter. We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.

Written by Ashley Hamer July 25, 2018

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