What Is Dark Matter?

Galaxies rotate so fast that the visible matter alone cannot provide enough gravity to hold them together — something unseen must be adding mass. This missing mass is called dark matter. It does not interact with electromagnetic radiation (light, radio, X-rays), making it invisible to telescopes. Its presence is inferred from gravitational effects: the rotation curves of galaxies, the bending of light around galaxy clusters (gravitational lensing), and patterns in the cosmic microwave background. Despite decades of searching, dark-matter particles have never been directly detected.