Something is holding galaxies together. It is not the matter we can see. Stars orbit galaxy centers too fast for the visible mass to explain. Clusters of galaxies are hotter than they should be. Gravitational lensing bends light more than visible matter alone would allow. The simplest explanation is dark matter: a form of matter that does not emit, absorb, or reflect light, but does exert gravity.

Dark matter makes up about 27 percent of the universe. It is not made of atoms. It does not interact electromagnetically, which is why it is invisible. It only interacts gravitationally, and possibly through the weak nuclear force. Candidates include WIMPs, axions, and sterile neutrinos. None have been detected yet.

Dark matter is essential to structure formation. Without it, gravity could not have pulled matter together fast enough to form galaxies. Simulations that include dark matter reproduce the observed cosmic web. Simulations without it do not.

The search is one of the biggest efforts in physics. Underground detectors, particle colliders, and telescopes all hunt for it. So far, nothing. But the gravitational evidence is overwhelming.