Dark Matter

Because I've been a bit down today, it seems fitting. It's also EXTREMELY cool.

The Universe is made up of 100's of billions of stars, presumably each the center of their own systems or part of nebulae or other systems and galaxies. Sometimes galaxies gather into clusters containing all sorts of visible matter (like stars, planets, asteroids, gases, and thermal energy). It turns out there is five times more material in clusters of galaxies than we would expect from the galaxies and hot gas we can see. Most of the stuff in clusters of galaxies is invisible and, since these are the largest structures in the Universe held together by gravity, scientists then conclude that most of the matter in the entire Universe is invisible. This invisible stuff is called 'dark matter', a term initially coined by Fritz Zwicky who discovered evidence for missing mass in galaxies in the 1930s. There is currently much ongoing research by scientists attempting to discover exactly what this dark matter is, how much there is, and what effect it may have on the future of the Universe as a whole.
In general, astronomers learn about the Universe by the electromagnetic radiation (or light) that we see from it. The light we see is in the form of radio waves, infrared, optical, ultraviolet, X-ray, and gamma-ray emission. But what if there is material in the Universe that does not glow? How will we ever know it is there? How can we tell how much of it there is? How do we know what it is? Dark matter has a gravitational pull on both the light and the sources of light that we can see. From the effects of "extra" gravity that we detect, we infer how much mass must be present.
The kinds of materials that we experience every day are made of atoms, which are composed of protons, neutrons, and electrons. We refer to this type of matter as "baryonic". Is the dark matter in the Universe made of the same stuff that we are familiar with, i.e., is it baryonic? Or is it something strange ... some kind of exotic new material, which we could call non-baryonic?
So far, it looks like there are both baryonic and non-baryonic types of dark matter. Some dark matter may be composed of regular matter (ie., baryonic), but simply not give off much light. Things like brown dwarf stars would be in this catagory. Other non-baryonic dark matter may be tiny, sub-atomic particles which aren't a part of "normal" matter at all. If these tiny particles have mass and are numerous, they could make up a large part of the dark matter we think exists.
Although dark matter was inferred by many astronomical observations, the composition of what dark matter is remains speculative. Early theories of Dark matter concentrated on hidden heavy normal objects, such as blackholes, neutron stars, faint old white dwarfs, brown dwarfs, as the possible candidates for dark matter, collectively known as MACHOs. Astronomical surveys failed to find enough of these hidden MACHOs. Some hard-to-detect baryonic matter, such as MACHOs and some forms of gas, is believed to make a contribution to the overall dark matter content but would constitute only a small portion.
Additionally, data from a number of lines of evidence, including galaxy rotation curves, gravitational lensing, structure formation, and the fraction of baryons in clusters and the cluster abundance combined with independent evidence for the baryon density, indicate that 85-90% of the mass in the universe does not interact with the electromagnetic force. This "nonbaryonic dark matter" is evident through its gravitational effect. At present, the most common view is that dark matter is primarily non-baryonic, made of one or more elementary particles other than the usual electrons, protons, neutrons, and known neutrinos. The most commonly proposed particles are axions, sterile neutrinos, and WIMPs (Weakly Interacting Massive Particles, including neutralinos).
Estimated distribution of dark matter and dark energy in the universeThe dark matter component has much more mass than the "visible" component of the universe. Only about 4.6% of the mass of Universe is ordinary matter. About 23% is thought to be composed of dark matter. The remaining 72% is thought to consist of dark energy, an even stranger component, distributed diffusely in space. Determining the nature of this missing mass is one of the most important problems in modern cosmology and particle physics. It has been noted that the names "dark matter" and "dark energy" serve mainly as expressions of human ignorance, much like the marking of early maps with "terra incognita".
An important property of all dark matter is that it behaves like and is modeled like a perfect fluid, meaning that it does not have any internal resistance or viscosity. This means that dark matter particles should not interact with each other other than through gravity, i.e. they move past each other without ever bumping or colliding.
Historically, three categories of dark matter candidates have been postulated. The categories cold, warm, and hot refer to the speed at which the particles are traveling rather than an actual temperature.

Cold dark matter – objects that move at classical velocities
Warm dark matter – particles that move relativistically
Hot dark matter – particles that move ultrarelativistically

Mentions of dark matter occur in some video games and other works of fiction. In such cases, it is usually attributed extraordinary physical or magical properties. Such descriptions are often inconsistent with the properties of dark matter proposed in physics and cosmology.