This answer was provided by physicists Rhett Herman of Radford University in Radford, Va. and Shane L. Larson of Montana State University in Bozeman, Mont.
Dark matter is just what its name implies; it is matter (or mass) in the universe that we cannot see directly using any of our telescopes. Our telescopes see not only visible radiation (constituting the spectrum of colors that our own eyes can detect), but other types of radiation as well.
Image: Royal Greenwich Observatory
Dark matter does not reveal its presence by emitting any type of electromagnetic radiation. It emits no infrared radiation, nor does it give off radio waves, ultraviolet radiation, X-rays or gamma rays. It is truly "dark." Cosmologists believe we can only see about 10 percent of the matter in the universe. Until they can accurately determine the mass of the universe, they will not know for sure whether it is expanding infinitely or will stop expanding at some point and collapse.
How, then, can we say with confidence that we know dark matter exists? The way in which dark matter reveals its presence to us is through the gravitational effect it exerts on luminous matter in the universe. ("Luminous" matter is the matter we can see with our telescopes.) The most obvious example of the gravitational effects of dark matter can be observed when looking at the rotation of galaxies.
To study galactic rotation, astronomers look at the emission line spectra of stars in each part of the galaxy. When the light from a star is observed using a diffraction grating or a prism, the starlight is separated into its true colors, in much the same way ordinary sunlight can be separated into the full rainbow of colors known as the visible spectrum.
The true colors constituting starlight separate into a series of light and dark lines in the visible spectrum, with each colored line corresponding to a specific wavelength of light. The specific wavelengths at which these lines occur are characteristic of the elements the stars contain. Thus, they can be used as an elemental "fingerprint" to identify a star's composition.
When a star emitting these line spectra is moving away from us, all of the wavelengths of the spectral lines are shifted to higher values than they would have been were the star stationary or moving side to side (neither towards nor away from us). All of the spectral lines are thus shifted towards the long wavelength part of the spectrum, or to the red end of the spectrum.
This shifting of the lines, known as a Doppler shift, towards the red end of the visible spectrum is the origin of the term "redshift." When a star has part of its motion towards us, the spectral lines are shifted to shorter wavelengths, or "blueshifted," towards the blue end of the spectrum. By measuring the shift in wavelength, researchers can calculate the precise speed of a star, either towards us or away from us.
When a galaxy is rotating, the starlight from stars on the side of the galaxy that is moving towards are blueshifted, while the starlight from the stars on the other side of the galaxy are redshifted. Thus, we can tell how fast and in what direction each individual star in the galaxy is orbiting about the center of the galaxy.
When stars orbit the center of a galaxy, their orbital speed is determined by the distribution of the mass contained within the galaxy. A graph showing the orbital speeds of the stars versus their distances from the center of the galaxy is known as the "rotation curve" for the stars in the galaxy.
If one takes all the luminous matter that can be seen in the galaxy (stars, gas and dust) and predicts the rotation curve using the well-known laws of gravitational physics discovered by Newton, the speed of stars should decrease in a predictable manner the father away they are from the center of the galaxy.
Looking at the rotation curves of galaxies, however, astronomers have found that rotational speeds do not fall off with distance as expected. Instead, the curves level off, and stars far away from the center of the galaxy move faster than expected. The only way to account for this observation is that a large quantity of matter which cannot be seen--dark matter--exists in the galaxies. To explain the astronomical observations, this dark matter must surround the galaxy in a large, spherical distribution known as a galactic halo.
Theoretical candidates for dark matter have been divided into two groups, dubbed MACHOs and WIMPs. The existence of MACHOs (Massive Astrophysical Compact Halo Objects) has been confirmed experimentally--recently in our own Milky Way galaxy. The nature and origin of MACHOs are currently a matter of great speculation and debate, but their masses and distributions have been measured by their gravitational effects. Proposals for candidate MACHOs include primordial black holes, as well as some types of new, exotic astrophysical objects whose properties have yet to be properly described.
The mass of the average MACHO appears to be around half that of our sun. But the number of MACHOs, although large, still appears to be too small to account for all the dark matter suspected to be present in the galactic halo. This fact has led astrophysicists to speculate on other possible dark matter forms, such as WIMPs.
WIMPs (Weakly Interacting Massive Particles) are exotic, massive elementary particles that do not interact strongly with matter. (Hence they have not been interacting with our detectors so we have not detected them yet). Because WIMPs do have mass, and there would be great numbers of them, their individually weak but collectively strong gravitational effects could account for part of the impact that dark matter has on the rotation curves of galaxies.
One possible class of candidates for dark matter are neutrinos. Just announced experiments using a detector in Japan, called the Super-Kamiokande, indicated that neutrinos may indeed have mass. Already, cosmologists are re-examining currently accepted theories.
Dark matter is known to exist through the gravitational effect it exerts on visible matter in the universe. As our astrophysical experiments become more sophisticated, and our understanding of large gravitational systems (galaxies and clusters of galaxies) grows, we will answer more of the questions that have faced us years.
New questions about the nature and origin of dark matter are continually being put forward, ensuring that this field will be exciting, dynamic and at the forefront of astrophysical research for years to come.