Dark matter formed when fast particles slowed down and got heavy, new theory says

Researchers propose a new theory for the origin of dark matter, the invisible substance thought to give the universe its shape and structure. Their mathematical models show that dark matter could have formed in the early universe from the collision of massless particles that lost their energy and condensed -- like steam turning into water -- into cold, heavy particles. They report that their theory can be tested using existing data -- these dark matter particles would have a unique signature on the radiation that fills all of the universe known as the Cosmic Microwave Background.