Dark Matter

Stars and galaxies are not all that there is. WMAP has shown that stars and galaxies account for only 4.6% of what the universe is made out of. Almost a quarter of the universe—24%—is made out of an unknown type of matter called dark matter. This type of matter does not emit electromagnetic radiation and can only be detected by its gravitational effects.

The first indications that the masses of the stars are not enough to explain our observations came in 1933. That year, the Swiss physicist Frank Swicky realized that the estimated masses of the stars in the Coma cluster could account for only a small fraction of the mass needed to prevent the stars from flying away. In the 1970s, the American Astronomer Vera Rubin and her collaborator W. Kent Ford of the Carnegie Institute observed the same phenomenon in individual galaxies. The mass of the stars in a galaxy accounted for only about 10% of the mass needed to keep the stars orbiting around the center of mass of the galaxy.

More accurate measurements performed since continue to show that dark matter is needed to explain not only the motions of stars in galaxies, but the bending of light around galaxies, the existence of the large-scale structure of the universe and even the tiny fluctuations in the cosmic microwave background radiation discovered by СОВЕ and corroborated by WMAP and the Planck satellite.

What is dark matter? We do not know yet. However, we can deduce several of its properties from our observations on its effect on regular matter. Since we cannot detect it with our telescopes, we know that dark matter does not emit electromagnetic radiation. From our knowledge of the formation of galaxies together with the measured density fluctuations in the cosmic microwave background radiation, we know that dark matter moves much slower than the speed of light. Much faster speeds would not have led to the formation of the galaxy structure we see today.

An early proposal for a dark matter particle was the WIMP or Weakly Interactive Massive Particle predicted by one of the supersymmetric theories. However, delicate searches for this particle since the 1990s have turned up empty. Moreover, WIMP theories predict a higher density of dark matter at the cores of galaxies and a higher concentration of satellite galaxies around the Milky Way than what is observed.

A second candidate for a dark matter particle was the axion, proposed by Steven Weinberg, then at Harvard University, and Frank Wilcsek of MIT to explain a broken symmetry mechanism in the strong force that Helen Quinn and Roberto Peccei of Stanford University had developed in 1977. Recent searches with the Large Underground Xenon experiment in South Dakota, the Particle and Astrophysical Xenon Detector in China, and at the Gran Sasso National Laboratory in Italy have shown no signs of these particles. But experimenters are not giving up. Leslie Rosenberg at the University of Washington and collaborators have developed the Axion Dark Matter Experiment (ADMX), an experiment sensitive enough to directly search for the best estimates of the axion mass. Rosenberg predicts that, over the next decade, ADMX could either detect axions or rule out all plausible versions.

Similar experiments are being developed at Yale University in the United States, in Australia, and in South Korea. There is also an astrophysical experiment in development at the Large Synoptic Survey Telescope (LSST) in Arizona that will observe several billion galaxies to understand the nature of dark matter. The LSST may be able to differentiate among the different dark matter candidates.

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