JON LOMBERG / SCIENCE PHOTO LIBRARY JON LOMBERG / SCIENCE PHOTO LIBRARY
Dark matter halo surrounding the Milky Way Galaxy (centre), artwork. These dark objects are MACHOs (massive astrophysical compact halo objects). They comprise ordinary matter and do not emit light. MACHOs include brown dwarf stars (small stars that failed to ignite), and the dark or faintly-glowing remnants of stars (like white dwarf stars, neutron stars and black holes). Other forms of dark matter have also been proposed. It is thought that only about 3 percent of the matter in the universe is visible. The dark matter is needed to account for the missing mass of galaxies (calculated through orbital motions), as visible stars do not comprise enough mass.
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