Dark matter has long remained elusive, but recent evidence suggests that it does exist, making up most of the matter in the universe. But what is it?
There are several theories as to what dark matter might be. The easiest explanation to understand, is that it is made up of non-luminous planets and dead stars (brown dwarf stars), or even black holes. Astrophysicists call these bodies MACHOs, which stands for massive compact halo object. Dark matter could also be made up of large clouds of gas that are essentially invisible because they are not near a source of light.
Another explanation is that the dark matter is made up of as yet unknown elementary particles. In a counterpoint to MACHOs indicating that physicists temper all the math and seriousness with a sense of humor, these hypothetical particles are called WIMPs, which stands for weakly interacting massive particles. WIMPs are thought to interact with other matter only through the weak and gravitational forces, but not the strong or electromagnetic forces. Lack of interaction with the electromagnetic force means that the particles do not use light in any way, and can't reflect or emit light or other frequencies of radiation. By far, these particles are considered to be the most likely explanation of dark matter even though none of these particles have been detected in nature or produced by scientists.
Recently, an indirect observation of dark matter was made in the Bullet Cluster, where a collision of two clusters of galaxies is taking place (pretty dramatic stuff!). As the two clusters collide, the dense material such as stars shoot through with little interference and as a result, little change in velocity. But the luminous gas clouds from the two clusters have a lot more interference from being more evenly spread out; the velocity of the gases are slowed in the collision. In a phenomena known as gravitational lensing, the portions of the galaxies containing the stars is shown to be more massive. It was thought that most of the mass of a galaxy was contained in it's gas clouds. But it is not the stars that are more massive than once thought - dark matter which does not interact with other matter except through gravity, would not alter it's velocity in the collision and would make it through without slowing. This observation has been widely regarded as proof that dark matter does exist.
