That's a great question, and one that astronomers can't answer. Even the smartest scientists in the world don't know what dark matter is. But they know it's out there. How?
All the planets, stars, galaxies and gas that can be seen today make up just 4 percent of the universe. The other 96 percent is made of stuff nobody can see, even with the most powerful telescopes. The strange stuff is actually two things: dark matter and dark energy. Astronomers know this stuff it out there because of the effects of gravity they can see on the objects that are observed.
If that has you feeling stumped, don't feel bad. Here’s how one really smart guy, science writer Richard Panek, thinks of it: "The overwhelming majority of the universe is: who knows?"
SpaceKids on SPACE.com provides simple, straightforward answers to really big cosmic questions. See more SpaceKids questions.
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Space.com is the premier source of space exploration, innovation and astronomy news, chronicling (and celebrating) humanity's ongoing expansion across the final frontier. Originally founded in 1999, Space.com is, and always has been, the passion of writers and editors who are space fans and also trained journalists. Our current news team consists of Editor-in-Chief Tariq Malik; Editor Hanneke Weitering, Senior Space Writer Mike Wall; Senior Writer Meghan Bartels; Senior Writer Chelsea Gohd, Senior Writer Tereza Pultarova and Staff Writer Alexander Cox, focusing on e-commerce. Senior Producer Steve Spaleta oversees our space videos, with Diana Whitcroft as our Social Media Editor.