Dark Matter is something that can't be seen, but its presence has been revealed by observations ofthe universe. Billions of years ago, the universe sprang forth with a cataclysmic big bang. As eons of time passes, this early universe slowly cooled and began to evolve. Eventually, stars, galaxies, and the rest of the visible universe took shape. The size of our universe is staggering. Our Sun is large enough to hold one million Earths. The Sun is an average-sized star. Our galaxy contains over 100 billion stars. That's more stars than there grains of sand on the average beach. But there's more. The universe is known to contain billions of galaxies. There is a lot of matter out there. But something seems to be missing. It appears that what we can see may not be all there is. Strong evidence is mounting that suggests there exists large amounts of dark matter in the universe. Scientists estimate that what we do see may only account for 10% of the mass of the universe. That means that 90% of the matter is invisible. Some estimates even place this number as high as 99%. Astronomers refer to this invisible mass as dark matter.
The evidence for dark matter lies with gravity. Gravity is the force or "glue" that holds the universe together. Everything in the universe is mutually attracted to everything else. Scientists have been able to calculate the total mass of the visible universe. They have also calculated the gravitational forces that hold the universe together. What they have found is that there does not appear to be enough visible matter to account for the mass that is required to gravitationally bind the universe together. In addition, dark matter can be detected through its gravitational influence on other objects, or even on light itself. It can affect the motion of stars and galaxies. Many galaxies have been found to be rotating much faster that they should. According to Einstein's theory of gravity, they should fly apart. But something unseen seems to be holding them together. Dark matter can also affect the path of light. In a phenomenon known as gravitational lensing, dense objects can cause the light of distant objects to bend around it. This can result in distorted images and duplicate images of stars and galaxies. We can tell that something is bending the light, but we can't tell what it is.
Some of the dark matter in our own galaxy may exist in what are known as Massive Compact Halo Objects, or MACHOs. MACHOs are believed to be made up of ordinary (baryonic) matter. They include objects such as planets, moons, brown dwarfs, dust clouds, white dwarfs, neutron stars, and black holes. They are too small to glow brightly, but they can be detected through their gravitational effect on light. Over the past few years, astronomers have detected several types of MACHO objects in our galaxy. While MACHOs could account for some of the dark matter, scientists believe that there is much more out there. Some of the matter may exist in what is known as non-baryonic matter. This type of matter would be composed mainly of exotic particles. These exotic particles have equally exotic names such as neutrinos, axions, supersymmetric dark matter, and WIMPs (Weakly Interactive Massive Particles). Although WIMPs have been predicted in theory, none have as of yet been detected. If these theories prove to be true, WIMPs could account for a significant portion of dark matter in the universe.
As the quest for dark matter continues, new instruments are helping us to gain more insight into this baffling mystery. The Hubble space telescope has provided valuable information about the size and mass of the visible universe. This information is an important first step towards estimating the true amount of dark matter. Hubble is helping us to understand the large-scale structure of the universe. The universe is not random. We have learned that galaxies are arranged in clusters and that these clusters are arranged into superclusters. The superclusters seem to be arranged in a sponge-like structure with large voids, or "bubbles' between them. Clearly, something must be responsible for these formations. X-ray telescopes such as the Chandra x-ray observatory have detected vast clouds of hot gas within clusters of galaxies. Scientists know that dark matter must be present in these areas, or the gas would escape from the cluster. Several new tools are currently under development that will ultimately allow us to "see" this dark side of the universe.