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Next Monday, October 31, many folks will celebrate the spooky season by dressing up and gathering with friends and loved ones to enjoy scares and snacks. In 2016, a group of particle physics communication specialists started Dark Matter Day to be celebrated alongside Halloween on October 31 of each year. Their hope is to raise awareness of dark matter—matter that supposedly makes up most of the universe but that can only be detected by its gravitational effects on other matter. Why is dark matter cool, and why should we get to know what it’s about?
According to Interactions.org, the group that started Dark Matter Day, dark matter makes up about 85 percent of the total mass of the universe and about a quarter (26.8 percent) of the universe’s total mass and energy. That’s a lot! Scientists also know that dark matter is everywhere; it does not absorb, reflect, or emit light and is therefore invisible; it binds galaxies; and it distorts space.
If dark matter is invisible, how did we learn that it’s actually there? We know how an object’s mass affects other objects with mass gravitationally. In other words, we know how to measure the effects of something’s gravitational pull based on effects we measure in other objects. When scientists detect and measure the effects of gravity on a mass when no other mass can be seen, that invisible mass is known as “dark matter.” Dark matter is called “dark” because we don’t know what it is—but we can see what it’s doing.
In this article, National Geographic offers this example of how scientists observe dark matter in action: According to standard physics, stars at the edges of a spinning, spiral galaxy should travel much slower than those near the galactic center, where a galaxy's visible matter is concentrated. But observations show that stars orbit at more or less the same speed regardless of where they are in the galactic disk. While puzzling, this makes sense if the boundary stars are feeling the gravitational effects of an unseen mass—dark matter.
So dark matter is unknown matter. We know something with mass exists, and we know that something isn't what the planets, stars, and other visible matter in the universe are made of. We also know that this matter accounts for most of the mass in the universe. Ultimately, if it wasn't there, the universe would behave much differently.
Learning to understand dark matter is critical to our understanding of not only the universe itself but its future. Share your new knowledge with curious friends and students and join in on the hunt for the most elusive substance in our universe. Interactions.org suggests finding an event near you or even planning your own, and the organization even offers resources to help you develop a program, find dark matter experts to speak at your event, help prepare for the event, and promote your event!