Introduction 
Introduction What is it? 
Introduction
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Prepare you mind, because things are about to get jumbo-funky. Astronomers and physicists offer a plethora of explanations for dark matter material. On the one hand, the dark matter could be baryonic matter such as ultra faint stars, black holes, cold gas, or dust scattered throughout the universe. All of these neglect to emit or reflect light which is visible in our electromagnetic spectrum. However, nucleosynthesis (the study of the origin of elements after the big bang) sets a limit to the number of baryons--particles of ordinary matter--that can exist in the universe.
Astronomers and Physicists have been able to construct two models of an acceptable "open" universe. The first is made of all baryonic matter--and matches all of our observations. However, this model violates the quantitative limit of baryonic matter determined by Nucleosynthesis. In the second model, they have taken baryons and exotic matter (non-baryonic particles) in quantities that add up to only 20 percent of the matter needed to close the universe. This model also parallels our observations. Thus, a satisfactory low-density universe has a few mystifying properties: most of the universes baryons would remain invisible, their nature unknown, but of greatest importance, most of the universe's matter must be exotic.
So the question then becomes, what is this exotic matter? There are three modern candidates for such dark matter: MACHOS, WIMPS, and Neutrinos.
MACHOs--short for MAssive Compact Halo Objects--could exist in huge amounts in the vast "halos" surrounding galaxies. Brown dwarfs form like stars but don't have enough mass to begin the nuclear fusion reactions that cause normal stars to shine brightly. These brown dwarfs are between the size of an ordinary star and a average planet. Most importantly, they qualify as MACHOs. As observed from Earth, many drift along the edges of the Milky Way and as they pass a bright object, they cause gravitational lensing. Two gravitational lensing experiments in 1993 reported strong evidence for the existence of MACHOs in the Milky Way.
Other possible MACHOs include planets--about a dozen of which have been discovered outside our solar system in the past couple of years. There are also unknown objectsthat cause the gravitational lensing. Both the planets and the are considered exotic because their exact creation method and composition is unknown.
Think about it, what if there was a planet that was made of completely unknown elements, where the people are yellow. The men might drink some kind of alcoholic liquid to sustain themselves, while the women develop blue hair. The adolescence might even have spiked hair and never age. Wouldn't that be weird? The point is, we simply don't know.
WIMPs--that's right, you guessed it--Weakly Interacting Massive Particles are also candidates for dark matter. These ghostly particles have thus far escaped detection, though their presence is predicted by theory of supersymmetry. Supersymmetry basically states that all known particles have a partner 'ino' particle. These particles are almost all short-lived, and most existed in large numbers only in the early universe, when the temperature was high enough. However, the lightest of these particles could still exist today. These supersymmetric particles are predicted to be extremely massive, relatively speaking; in theory, they would have masses of perhaps 10 to 100 times that of the proton. Cosmologists like these WIMPs because they are relatively heavy and therefore, slow moving. If this is true, then they could have been the primordial "seeds" around which ordinary matter collected to form galaxies. At the same time, they don't interact with radiation, and hence, would not alter the smoothness of the cosmic background radiation. The photino (an exotic and supersymmetric particle) has been postulated to exist in sufficient quantity to account for dark matter. The problem is that there is no guarantee that these particles even exist. Experiments are currently underway in search of WIMPs such as the photino.
Why is it so special? Allow us to explain. In laboratories, physicists have been able to accelerate particles to 99.998 percent the speed of light. The particles are then rammed into a lead target. What happens? Well the particle obviously stops--dead in its tracks. Every so often, there are some kinds of particles that seem to just pass right through the target--leaving no wake and causing no reaction with the lead. Also, there is a loss of energy when calculating the physics of this collision--or lack there of. Energy must be conserved (equal amounts of energy before and after an event), so where did the extra energy go? It was carried onward by the neutrino! These surviving particles are known as neutrinos. So why are they so special? Simply put, we have evidence and data that proves their existence.
One billion neutrinos exist in the universe for every proton or electron; they add a colossal amount of mass to the dark matter total. In fact, there are so many neutrinos left over from the big bang that a neutrino mass of even 50 eV (or one ten-thousandth of the mass of an electron) would be sufficient to close the universe. However cool this may seem, there is currently heated debate over whether a neutrino has any mass at all. When they were first theorized in the 1930's by physicist Wolfgang Pauli to explain radioactive beta decay, neutrinos were thought to have no mass at all and travel at the speed of light. If neutrinos indeed lack mass, then there would be a vast amount of matter in the universe that is currently unidentified. Experiments are underway to determine the mass of the neutrino. Some of the results seem to indicate a very infinitesimal mass (millions of times less than a proton) and near the speed of light travel. However, at present, results are inconclusive. At this point, there are three types of neutrinos: electron neutrinos, muon neutrinos, and tau neutrinos. (These are also known as the three flavors.) If indeed these particles have mass, than the electron neutrino would be the lightest, the tau neutrino the heaviest, and the muon neutrino somewhere in the middle. (Remember, when we say heavy, we mean relatively speaking.) Neutrinos are all around you, even now in that very room. You can't see them or feel them, but millions--even billions--zoom through your body every day. Because they have such a tiny mass (if at all) they fly through your body like it isn't even there. Even more bizarre, in your body's lifespan the neutrinos will cause few, if any, reactions.
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