This is something I wrote a while back, if it is too long, that’s ok, don’t read it. 
When thinking of our universe, there are a few things that people normally think of: galaxies, planets, gas, nebulas, and maybe even black holes. All these objects seem pretty normal and complete. Astronomers were perfectly happy with trying to figure out how all the above-mentioned things worked and interacted together when a large kink came into all the theorizing. Why were the velocity curves of galaxies showing that there should be more mass than was seen in all the “normal” or baryonic matter? All the data did not make sense. To most people, it still does not. After analyzing data from velocity curves and studying the dynamics of how galaxies interact with each other, it was determined that approximately 90% of the mass of the entire universe can not be seen. What does that mean and how is that possible? This indicates that previous notions about what the universe was made of must be rethought. The only way for this to be possible is if there is some form of baryonic matter that we cannot see or some new from of exotic matter that has not been thought of before. A few of the current theories about what the missing matter is in the universe are MACHOs, WIMPs, neutrinos, and annihilating particles. Until final proof about what dark matter is virtually any of the theories could be right, wrong, or a mixture of both. As time progresses theories will change, fail, and new ones will arise. Current theories are very interesting and many astronomers feel very strongly about one theory or another. Evidence for and against should be looked at in a case-by-case matter in order to formulate new postulations that can in turn be tested and researched.
After realizing that the majority of the mass of the universe is unseen, it became the task of many astronomers to figure out what this missing mass is and where it resides. There has been much debate about what the mater really is between astronomers. Those supporting MACHOs, Massive Compact Halo Objects, and those supporting WIMPs, Weakly Interacting Massive Particles, have strong ideas and both hold true to their theories and will continue to do so until they have been completely disproved. MACHOs are exactly what their name suggests, massive objects that reside in a galaxy’s halo. The halo is a spherical region centered at the middle of a galaxy’s bulge. Globular clusters (large groupings of up to hundreds of thousands of old stars within in a small volume of space) that are already seen in this region of a galaxy are letting astronomers know that matter can and does exist in that area. Velocity profiles suggest dark matter is present and lead astronomers to believe that there could be a large amount of mass in the halo region. Since the globular clusters in the halo, in addition to the stars and other visible material in the plane of the galaxy, are not enough mass to account for the velocity curves, there must be some other mass somewhere in the galaxy, including the halo. MACHOs are what astronomers have come up with to account for this matter.
What is needed is normal baryonic matter that could be very massive and reside in the halo. The first thing that comes up is a star that did not have enough mass to ignite. A brown dwarf is a star similar to the sun, but less massive, made of hydrogen but could not ever get the nuclear furnace going. If they aren’t brown dwarfs, MACHOs are most likely black holes. (Lewin 1997) A black hole is a star that has collapsed down from sizes larger than our sun to smaller than a small rock. A black hole is so dense that not even light can escape from it. Black holes have been theorized to exist in many places and there is a theory that suggests that there could be many so-called primordial black holes left over from the Big Bang. A black hole is a great example of what dark matter could be as no light can escape from it. As a result they are impossible to directly detect (but can be detected in other ways). So how do astronomers go about detecting objects that give off no or very little light? The first method is to just look into the sky. It is much harder because objects like brown dwarfs are very faint. With newer technologies such as adaptive optics and newer generation space telescopes it is easier to directly observe these objects. Data taken from the Hubble Space Telescope suggests that brown dwarfs only make up around 6% of the matter in the halo. This is a good start, but obviously not the 90% that should be there. Consequently astronomers must move on to another detection method. The primary method being used and studied currently is strong gravitational lensing. Sound like optics? It is actually a very good analogy to optics in that mass, and therefore an associated gravity, can bend light to a point, effectively lensing starlight.
As seen in the image (Fig 1), light from a very distant galaxy emits light in all directions, some straight at us, some away from us. Since matter causes gravity, very massive objects are able to bend light. The next image (Fig 2)
shows the effects of this light bending.
Fig 2: Gravitational Lensing of a Galaxy
Instead of seeing one bluish galaxy in the center, a bluish (because of the color of the galaxy) light or smudge will show up in a ring pattern around the center. So what does this do for the search for dark matter? After studying the images of candidates for this lensing, density maps can be plotted. By studying how the light has been bent and by how much, these plots are created. When the mass distribution has come out of that it can be compared to what is actually seen visually. If there is a discrepancy between how much mass we physically see there and how much mass should be there to cause the lensing, dark matter has been found (in theory). From this lensing exact placement of the dark matter can be determined even if it is not known exactly what it is (i.e. a back hole, brown dwarf, etc.). In the pictures the dark matter of the central cluster outweighs all of the combined mass of the cluster’s galaxies by 250 times. Similar to strong gravitational lensing, weak lensing is another method to detect dark matter MACHOs. The presence of an unseen object can distort the light coming directly from a galaxy. The warping of a distant galaxy is a good indication that there is some gravitational force acting on the light between the galaxy and the earth. There are other methods for detecting MACHOs but the three mentioned above at current time are the most interesting and the most promising for actual detection and finding much of the missing mass. (Bell Labs 1999)
The next interesting possibility are new, exotic particles called WIMPS (Weakly Interacting Massive Particles). WIMP theory is currently the most popular among astronomers. To many it has the most promise to account for all the missing matter. Astronomers suggest that these new particles do not interact with matter like other particles do. However, if they have a slight amount of mass (on order of a few hydrogen atoms), because of suggested amounts, they could account for much of the missing mass in the universe. (Lewin 1997) The hard part, once again, of this theory is to figure out how to detect particles that very rarely interact with normal matter. Particle physicists now get involved to help astronomers determine what these particles are and how to detect them. Since WIMP interactions are very rare in normal matter the idea is to set up a very sensitive instrument that can detect the slightest interactions and also be able to get rid of data from cosmic rays that have slightly similar interactions. One such project involves cooling a crystal to near absolute zero (to reduce motion of the atoms) and registering interactions in the crystal by detecting a heat rise. (Miller 1995) So detection of WIMPS will come with updating and inventing newer and better detectors that can discriminate between the new, rarely interacting, particles and particles that we already know the physics for.
Astronomers have opened up a very popular and extremely interesting new area of study with advances in dark matter research. MACHOs and WIMPs are only two of the theories surrounding the enigma of where all the mass is. Even if it can’t be decided which theory is correct, the drive to figure it out will keep bettering theories and inspiring people to come up with new ones. Down the road it will probably be figured out that the dark matter in our universe is made up of many different things, probably incorporating aspects of many different theories. The striving for understanding about how our universe will ultimately lead to better and better theories that will hopefully bring us closer to the truth. It is now known that dark matter is out there, but now the task is to pinpoint what it is.
References
“Bell Laboratories Physical Science Research: Dark Matter” 1999. http://www.bell-labs.com/org/physicalsciences/projects/darkmatter/darkmatter1.html (29 Nov. 2000)
Lewin, J.D. “CLRC RAL Open Days 1998. Dark Matter.” April 1997. http://hepwww.rl.ac.uk/ukdmc/pop/galactic-dm-exhibit.html (4 Dec. 2000)
Miller, Chris. “Cosmic Hide and Seek: the Search for the Missing Mass.” 1995. http://www.eclupse.net/~cmmiller/DM/ (4 Dec. 2000)
EDITOR’S NOTE: This post is from a previous blog so the original comments no longer exist.