LYNETTE COOK / SCIENCE PHOTO LIBRARY LYNETTE COOK / SCIENCE PHOTO LIBRARY
Dark matter. Illustration of a magnifying glass looking at dark matter made of subatomic particles, such as neutrinos or Weakly-Interacting Massive Particles (WIMPs), against a background of spiral galaxies. Dark matter is a theoretical concept used by cosmologists to explain why the stars in a galaxy stay together. The gravitational force within a galaxy can be determined by its rotational period. The mass of the stars can be calculated by measuring its luminosity. Cosmolo- gists estimate that only about 10% of the gravita- tional force holding galaxies together comes from visible mass. Some think that the other 90% comes from dark matter made of subatomic particles.
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