Galaxies in our universe seem to be achieving an impossible feat. They are rotating with such speed that the gravity generated by their observable matter could not possibly hold them together; they should have torn themselves apart long ago. The same is true of galaxies in clusters, which leads scientists to believe that something they cannot see is at work. They think something we have yet to detect directly is giving these galaxies extra mass, generating the extra gravity they need to stay intact. They call this mysterious stuff dark matter.
Unlike normal matter, dark matter does not interact with the electromagnetic force. This means it does not absorb, reflect or emit light, making it extremely hard to spot. In fact, scientists have been able to infer the existence of dark matter only from the gravitational effect it seems to have on visible matter.
Dark matter seems to outweigh visible matter roughly five to one, making up more than 80% of all the matter in the universe. Scientists think that dark matter particles were some of the few types of particles created in the big bang that are stable enough to still be around today.
Experiments at the Large Hadron Collider (LHC) may provide more direct clues about dark matter. Many theories say the dark matter particles would be light enough to be produced at the LHC. If they were created at the LHC, they would escape through the detectors unnoticed. However, they would carry away energy and momentum. Scientists could infer their existence from the amount of energy and momentum missing after a collision.
Dark matter candidates arise frequently in theories that suggest physics beyond the Standard Model, such as supersymmetry and extra dimensions. If one of these theories proved to be true, it could help scientists gain a better understanding of how our universe is composed and, in particular, how galaxies hold together.