What is dark matter?
Dark matter is an invisible form of matter that does not give off, absorb or reflect light, but its gravity holds galaxies together. It makes up about 85 percent of all matter in the universe.
In the 1930s, Swiss astronomer Fritz Zwicky noticed that galaxies in a cluster were moving too fast to be held together by the visible matter alone. In the 1970s, American astronomer Vera Rubin found that stars at the edges of spiral galaxies orbit as fast as those near the center. Both observations suggested a large amount of unseen mass.
Further evidence comes from gravitational lensing, where the gravity of galaxy clusters bends light from objects behind them more than visible matter can explain, and from patterns in the cosmic microwave background. Together, these suggest that dark matter outweighs ordinary matter by about five to one.
What could it be?
No one knows what dark matter is made of. Many scientists think it consists of undiscovered particles, such as WIMPs or axions. Large detectors deep underground and experiments at particle colliders are searching for them, but none has been confirmed yet.
Did You Know?
The Vera C. Rubin Observatory in Chile, named after the astronomer who found key evidence for dark matter, released its first images in 2025.