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More than Meets the Eye

What is Dark Matter Made Of?

Inflationary theory predicts that the universe is flat--that the average density of matter in the universe exactly equals the critical density required to close the universe. The matter that we can see is, at most, 10 percent of the critical density. And, even if inflation is not correct and we do indeed live in an "open universe," there's still a lot of unseen matter out there. This unseen matter, called dark matter because it does not reflect light, keeps spiral galaxies from flying apart and moves galaxy clusters and even superclusters in a great path across space.

Baryonic Dark Matter

Particle physicists and astrophysicists continue to speculate on the nature of dark matter. Some dark matter may simply be ordinary, or "baryonic" matter made of protons, neutrons and electrons that fails to emit radiation detectable on Earth.

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 galaxies.
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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.

MACHO Observation

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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Non-baryonic Dark Matter

Cosmologists are exploring another possible source of dark matter: exotic, non-baryonic particles. These particles come in two flavors: cold dark matter and hot dark matter. Cold dark matter refers to extremely massive particles that move very slowly. These particles have been dubbed WIMPs (Weakly Interacting Massive Par ticles) by physicists, who seem to never miss an opportunity for a joke. Scientists have postulated the existence of photinos--partners of photons--with an expected mass 10 to 100 times that of a proton; axions, carriers of force that have mass, or even quark nuggets, odd non-baryonic aggregates of quarks. None of these particles have been detected, either in space or in particle accelerators.

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.

Clues from the Computer

Even though we don't know what forms dark matter takes, scientists are convinced it exists. If it didn't, present assumptions about the origin and fate of the cosmos would fall apart.

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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Copyright © 1995, The Board of Trustees of the University of Illinois

NCSA. Last modified 11/2/95.