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dark matter

Astronomical observations of galaxies and galaxy clusters as well as comparison of observations with the predictions of the big bang models show that only about 15 percent of matter in the universe announces its presence by giving off light or other kinds of electromagnetic radiation. The other 85 percent of the mass are supplied by dark matter, and cosmologists have convincing evidence that most of that dark matter is not in the form of the usual atomic constitutents protons and neutrons. The exact properties of these unusual matter particles are not yet known, other than that they do not interact with ordinary matter and radiation other than by gravitational influence.