If physicists had a “Most Wanted” list, dark matter particles would be right at the top.
Dark matter is an invisible material that lurks throughout the cosmos. In fact, it makes up about 85 percent of the matter in the universe. Unlike the ordinary matter inside you, your computer, the planet and all the stars in the sky, dark matter doesn’t produce or reflect any light. For decades, physicists have tried to identify the particles that make up this mysterious substance. But so far, all searches have come up empty.
Hold on, you might say. If dark matter is invisible, how do we even know it exists? Dark matter is detectable because of the gravitational tug it exerts on visible objects. This is similar to the way you can tell it’s windy outside without being able to see wind. You know there’s wind because you can see it rustling the leaves on trees.
The first clues that dark matter exists came in the 1930s. An astronomer named Fritz Zwicky peered at a distant swarm of galaxies and found something odd. The galaxies were moving fast. In fact, they were moving so fast that the galaxy cluster should fly apart. So there must have been some unseen material hiding among the galaxies, holding the cluster together with its gravity.
In the 1970s, astronomer Vera Rubin found that stars swirl around spiral galaxies much faster than expected. At such high speeds, these stars should fly apart. To avoid shredding themselves, galaxies must be held together by dark matter’s gravity.
Most scientists are now convinced that dark matter exists. But they still have no idea what it is. Many different types of particles have been proposed to explain dark matter. Yet experiments designed to search for those particles have so far only ruled out contenders. As a result, some physicists have an alternate idea. Maybe dark matter doesn’t exist at all. Maybe at very large scales, gravity simply behaves in strange ways we do not yet understand.
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Spin in this Milky Way bar may show cosmic dark matter does exist The gravitational pull of dark matter may be slowing down a rotating bar of stars at the center of our Milky Way galaxy. (7/19/2021) Readability: 7.4
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Having a hard time imagining how scientists “see” invisible dark matter? Try this at-home experiment from NASA. Drop some beads or other small items into two plastic bottles, then fill one bottle up with water. Like dark matter, the water is transparent, but its effects can still be detected. You can see this when you compare how the motion of visible objects, such as beads, differs between the two bottles.