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One source of baryonic dark matter is the recently discovered primordial helium. The helium, along with the hydrogen that almost surely accompanies it, is scattered throughout the intergalactic medium. Scientists estimate that this primordial matter equals or exceeds all of the baryonic matter previously accounted for.
Other candidates for baryonic dark matter have been dubbed MACHOs (Massive Compact Halo Objects), which may include small, dim stars called red dwarfs, Jupiter-size planets that don't initiate nuclear reactions, and even black holes. A team of astronomers using the world's most powerful telescope, the Keck Observatory in Hawaii, recently made the first confirmed sighting of a brown dwarf, an exceedingly dim object somewhere between a planet and a star in size.
Missing Red Dwarfs
But astronomers using NASA's Hubble Space Telescope detected a paltry
number of red dwarfs in the Milky Way's halo. They've ruled out red dwarfs as
significant contributors to the dark matter in the Milky Way and, by extension, other
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Scientists using powerful land-based telescopes are using a technique called "gravitational lensing" to detect MACHOs in the Milky Way. Einstein's General Theory of Relativity shows that the fabric of spacetime is warped around massive objects; any light passing through that warped spacetime should therefore be bent. MACHOs could act as gravitational lenses by diffracting the light rays from more distant objects as they journey to the earth.
Astronomers focusing on gravitational lensing effects from stars in the Large
Magellanic Cloud--a galaxy in the Local Group--detected very few MACHOs
in the halo of the Milky Way but, surprisingly, more than expected in the center.
Still, there don't seem to be enough MACHOs to account for the internal
motions and relative velocities of galaxies.
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Hot dark matter, on the other hand, is made up of light-weight particles that move near the speed of light. One of the most likely candidates is the neutrino. Previously thought to possess no mass, recent experiments indicate that some types of neutrinos may actually have between one million and one thousandth the mass of an electron. Now, that's a mighty tiny particle, but our universe is absolutely swarming with neutrinos created during the great matter-antimatter annihilation that took place shortly after the Big Bang. If they do indeed have mass, they could easily account for the "missing" dark matter necessary for a flat universe.
Supercomputer simulations of the formation of galaxies and clusters rely on educated guesses about the nature and amount of dark matter. By trying to simulate observed structures, cosmologists learn more not only about how galaxies form, but also what amounts and types of dark matter are needed to explain cosmic evolution.
Clearly, though, dark matter -- what it is and how prevalent it is in the universe -- remains one of cosmologies greatest unsolved mysteries.
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