Definition
Dark matter is a mysterious kind of matter that doesn't give off, absorb, or reflect light, so no telescope can see it directly. But it has mass, and it reveals itself only through gravity. There is about 5 times more of it in the universe than all the ordinary matter we can observe, including stars, planets, and gas combined.
Galaxies Are Spinning Too Fast
Think about a merry-go-round at an amusement park. When it spins too fast, you feel like you'll be flung right off. Galaxies are the same. Hundreds of billions of stars swirl around together in a giant disk.
Most of the visible matter is bunched up in the middle. So the farther a star is from the center, the slower it should orbit. That's how our solar system works: planets far from the Sun move more slowly. But when astronomers actually measured the speeds of stars on the outer edge of galaxies, they didn't slow down. They were moving at nearly the same speed as the stars closer in.
At that speed, the outer stars should scatter and fly out of the galaxy. Yet galaxies don't fly apart. They hold their shape firmly. This means that, besides the stars and gas we can see, something we can't see is gripping the stars tightly with enormous gravity.
How Do You Find Something You Can't See?
A clear glass marble is hard to spot. But hold it in front of some printed text, and the letters look warped. That's how you know the marble is there. We find dark matter in much the same way.
According to Einstein's theory of relativity, very massive objects bend the space-time around them. So light coming from behind them gets bent, as if passing through a magnifying lens. This is called gravitational lensing.
When astronomers look at the sky, they sometimes see light from a galaxy in the background stretched into huge arcs or rings around a galaxy cluster. The visible stars and gas in the cluster can't explain that much bending. It's strong evidence that a huge clump of mass that gives off no light, dark matter, is hiding there.
A Little More Precisely
The "dark" in dark matter doesn't mean black. It means it gives off no light, so we can't see it. It also hints that we still don't know what it is. Since it doesn't interact with light at all, it's better to think of it as perfectly transparent rather than black.
Many scientists think dark matter is most likely a new fundamental particle that interacts only very weakly with other matter, apart from gravity. It barely interacts with forces like electromagnetism or the strong nuclear force. Several candidates, such as WIMPs and axions, are being studied, but its identity is still unknown. Because it almost never bumps into other matter, it probably passes through our bodies and the whole Earth all the time without us feeling a thing.
Look at the makeup of the whole universe today. The ordinary matter we can see and touch is only about 5%. Dark matter makes up about 27%. Dark energy, which makes the universe's expansion speed up, makes up the remaining 68% or so. Most of the universe is still full of secrets we don't understand.
Common misconceptions
Dark matter is a substance that absorbs light and looks black, or it's just a black hole.
Dark matter doesn't absorb or block light. Light simply passes straight through it, so it's transparent. Many scientists think it's an unknown particle that doesn't interact with light electromagnetically. Some researchers are studying the idea that tiny primordial black holes might make up part of it, but that doesn't mean the black holes we know of explain all of it.
Where you meet it
Stars at the edge of a galaxy orbit fast and stay stable instead of flying off. That's one way we know dark matter exists.
Light from behind a giant galaxy cluster appears bent into curves, as if passing through a magnifying glass. This gravitational lensing is a classic piece of evidence.
In one sentence
Dark matter doesn't react with light, so we can't see it. But its powerful gravity holds stars and galaxies together, making it the invisible frame that supports the universe.