Dark Matter

Dark matter is a mystery substance that seems to fill most of the universe. Scientists cannot see it. They cannot touch it. No telescope has ever taken a picture of it. But astronomers are sure it is there, because its gravity pulls on things they can see. Dark matter is one of the biggest unsolved puzzles in science.

The first clue came in the 1930s. A Swiss astronomer named Fritz Zwicky was studying a giant group of galaxies. The galaxies were moving so fast that they should have flown apart. Something was holding them together, but he could not see what. In the 1970s, an American astronomer named Vera Rubin found the same problem inside single galaxies. Stars at the edges of galaxies were spinning way too fast. Regular gravity from the visible stars was not enough to keep them in place.

The best answer scientists have is that galaxies are wrapped in huge, invisible clouds of dark matter. The extra gravity from these clouds holds everything together. Our own galaxy, the Milky Way, sits inside one of these clouds. The cloud is much larger and heavier than the bright disk of stars we can see.

So what is dark matter actually made of? Nobody knows yet. It does not seem to be made of atoms, like everything else around you. It does not give off light, heat, or radio waves. It barely interacts with normal matter at all. Dark matter particles may be drifting through your body right now, and you would never feel a thing.

Scientists have built huge experiments to try to catch a dark matter particle. Some sit deep underground in old mines, shielded from other kinds of radiation. So far, none of them has found a clear signal. Some scientists think dark matter is made of a new kind of particle that has not been discovered yet. Others think our understanding of gravity itself might be wrong at very large distances. The debate is still going.

Dark matter is not the same thing as dark energy, even though the names sound alike. Dark energy is a mystery force that is pushing the universe apart and making it expand faster. Dark matter pulls things together with gravity. Together, these two unknowns make up about 95 percent of the universe. The stars, planets, and people we know about are only the last five percent. Almost everything out there is still waiting to be understood.