The universe is made up of ordinary matter, dark matter and dark energy. Only 4.6 percent of the universe is made from ordinary atoms, the other 95.4 percent is composed of matter that humans cannot see.
Until roughly 1985, astronomers believed that the universe was composed of ordinary matter, such as protons, neutrons and electrons. However, when astronomers made accurate measurements of cosmic microwave background fluctuations and measured the motions of stars and gas, it was discovered that there were not enough atoms to account for the universe's mass. Researchers found that the Milky Way and other galaxies like it had 10 times the mass than could be caused by visible dust and stars. This mass discrepancy was explained by the introduction of matter that exerted gravitational pull but did not emit or absorb light, called dark matter.
Several speculations have been made on the nature of dark matter, including brown dwarfs, supermassive black holes and a possible new form of matter. Both brown dwarfs and supermassive black holes are extremely hard to detect due to their lack of luminosity. If enough of these objects were observed with gravitational lensing, it could account for the universe's missing mass. New forms of matter are being investigated using supercolliders to try and observe the creation of particles that share properties with dark matter.