The Invisible Structure
Much of the matter inferred in the universe does not shine, reflect or absorb light in the ordinary way. We know it mainly through gravity.
Dark matter is the name astronomers give to a component of the universe whose presence is inferred from gravitational effects. It does not mean that the material is simply ordinary matter hidden in darkness; observations indicate that there is more gravitational mass than visible matter alone can explain.
Galaxies move too fast
Stars orbiting within galaxies move at speeds that cannot be explained by the gravity of the visible stars and gas alone. The simplest interpretation is that galaxies sit inside much larger halos of unseen mass.
Gravitational lenses
Gravity bends light. When a massive structure lies between Earth and a distant galaxy, the foreground gravity can distort, magnify or multiply the background image. By measuring these distortions, astronomers can map the distribution of mass, including mass that emits little or no light.
A cosmic framework
Dark matter also helps explain how large-scale structure developed. In the standard cosmological picture, dark matter's gravitational influence helped matter clump together, providing a framework in which galaxies and clusters could form as the universe evolved.
What is it made of?
That remains an open question. Many candidate particles have been proposed, but no single dark-matter particle has yet been established as the answer. Astronomers therefore combine astrophysical observations with laboratory searches and particle-physics experiments.
Dark matter is one of astronomy's central unsolved problems precisely because its effects are so broad. It appears to shape the universe on enormous scales while remaining elusive at the level of direct detection.