What are scientists referring to when they say "Dark Matter" is the most abundant substance in the universe?
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It's the matter that provides the gravitational force that stops our galaxies from being flung apart by angular momentum.  It's called dark because we can't see it, but we know it's there because it bends light
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No, not really.\n\nEven high school labs now do light experiments that show photons are quanta.  One simple experiment, is with the photometer that emits electrons when struck by light.  The electrons are like billiard balls and the photons are the cue balls striking them to send them on their merry way.\n\nIf light were simply a ray (a force field), the amount of electrons emitted would be proportional to the strength of the light beam.  But they aren't and the number of electrons actually follow a step-function where at a given threshold of light, the electrons suddenly burst forth after none up until that threshold.\n\nThe photon concept, as bundles of energy, is the result of what I would call good physics.  Good physics is physics that starts with the physical phenomenon and then derives equations to explain it.  Bad physics, like string theory, starts with the math and then tries to explain a physical phenomenon, like strings.\n\nAs to your last assertion, think about it...would even physicists, who are renowned for their weird jargon, call something a "matter," which is a synonym for "mass" if it were not a mass?  No, dark matter is a mass...more correctly a mass-energy (due to E = mc^2).  DM accounts for about 96% of all the mass-energy in our known universe.  \n\nUntil just recently, DM has not been observed, which is why they called it dark.  But a recent observation of two galaxies colliding has seen the effects of the dark masses surrounding those two galaxies also colliding.  So, there is now observable evidence that the dark matter exists...the theory has been validated. [See source.]
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Matter that is in space but is not visible to us because it emits no radiation by which to observe it. The motion of stars around the centers of their galaxies implies that about 90% of the matter in a typical galaxy is dark. Physicists speculate that there is also dark matter between the galaxies but this is harder to verify.\nhttp://pdg.web.cern.ch/pdg/cpep/glossary.html\n\nMatter that does not emit enough light or other radiation to be observed directly. Most of the matter in the universe is believed to be of this type. Cold dark matter had a low velocity compared to the speed of light during the epoch of recombination. An example would be elementary particles with mass about equal to that of a proton or higher. Hot dark matter had a high velocity (near the speed of light) during the epoch of recombination. An example would be light elementary particles.\nhttp://www.nap.edu/readingroom/books/cosmology/glossary.html\nIn cosmology, dark matter consists of matter particles that cannot be detected by their emitted radiation but whose presence can be inferred from gravitational effects on visible matter such as stars and galaxies. Estimates of the amount of matter in the galaxies, based on gravitational effects, consistently suggest that there is far more matter than is directly observable. ...\nhttp://en.wikipedia.org/wiki/Dark_matter
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A galaxy is a huge gravitationally bound system of stars, interstellar gas and dust, plasma, and (possibly) unseen dark matter. Typical galaxies contain 10 million to one trillion (107 to 1012) stars, all orbiting a common center of gravity. In addition to single stars and a tenuous interstellar medium, most galaxies contain a large number of multiple star systems and star clusters as well as various types of nebulae. Most galaxies are several thousand to several hundred thousand light years in diameter and are usually separated from one another by distances on the order of millions of light years.\n\nAlthough so-called dark matter appears to account around 90% of the mass of most galaxies, the nature of this unseen components is not well understood. There is some evidence that supermassive black holes may exist at the center of many, if not all, galaxies.\n\nIntergalactic space, the space between galaxies, is filled with a tenuous plasma with an average density less than one atom per cubic meter. There are probably more than a hundred billion (1011) galaxies in the visible universe.
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Anti-matter colliding with matter is the most powerful mass to energy reaction.  The problem is that we have not yet managed to produce anti-matter in large enough amounts to make it available and "stable" enough to be used safely.
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So far science doesn't know what so-called dark matter is. Dark matter is the name for some exotic type of mass that is required to explain the motions of large structures in space, like galaxies, clusters of galaxies, etc.,. At this time, we think that dark matter accounts for about 23% of the universe's mass, while regular matter can only account for about 4%. The remaining 73% is thought to be what's called 'dark energy,' the totally unknown feature that is causing the universe's expansion rate to speed up.
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Fact.  Sort of.\n\nWhat is a fact is that every single repeatable measurement of large scale mass shows a very large discrepancy between how much dynamical mass is there (that is to say how much gravity says is there), and how much we can account for using every trick we know (and some we don't).\n\nGalaxy rotation, galaxy cluster temperature, WMAP data, you name it, everything we do to measure dynamical mass of very large objects comes back with us missing large amounds of accountable mass.\n\nEither there is some form of "dark matter" or we are encountering some new aspect of Gravity.  Right now Dark Matter is winning out because no one has been able to come up with a new twist on gravity that gets things right.
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In cosmology, dark matter refers to hypothetical matter particles, of unknown composition, that do not emit or reflect enough electromagnetic radiation to be detected directly, but whose presence can be inferred from gravitational effects on visible matter such as stars and galaxies. The dark matter hypothesis aims to explain several anomalous astronomical observations, such as anomalies in the rotational speed of galaxies (the galaxy rotation problem). Estimates of the amount of matter present in galaxies, based on gravitational effects, consistently suggest that there is far more matter than is directly observable. The existence of dark matter would also resolve a number of inconsistencies in the Big Bang theory, and is crucial for structure formation.
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In cosmology, dark matter refers to hypothetical matter particles, of unknown composition, that do not emit or reflect enough electromagnetic radiation to be detected directly, but whose presence can be inferred from gravitational effects on visible matter such as stars and galaxies. The dark matter hypothesis aims to explain several anomalous astronomical observations, such as anomalies in the rotational speed of galaxies (the galaxy rotation problem). Estimates of the amount of matter present in galaxies, based on gravitational effects, consistently suggest that there is far more matter than is directly observable. The existence of dark matter would also resolve a number of inconsistencies in the Big Bang theory, and is crucial for structure formation.\n\nIf dark matter does exist, it has vastly more mass than the "visible" part of the universe. Only about 4% of the total mass in the universe (as inferred from gravitational effects) can be seen directly. About 23% is thought to be composed of dark matter. The remaining 73% is thought to consist of dark energy, an even stranger component, distributed diffusely in space, that probably cannot be thought of as ordinary particles. Determining the nature of this missing mass is one of the most important problems in modern cosmology and particle physics.
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If you mean hottest as most POPULAR , then it would be the search for Quantum Gravity ( i.e string theory, loop quantum gravity etc. )\n\nIf you mean hottest as in TEMPERATURE, then it would be the study of the Big Bang, presumably the hottest the universe ( or anything for that matter ) has ever been !\n\nIf you mean coolest as in most INTERESTING, then it would be Nanotechnology, with its many far reaching implications on everyday life.\n\nIf you mean coolest as in TEMPERATURE, then obviously Superconductivity ( which occurs only at very low temperatures ) is THE coolest !\n\nhope this answers your cool question ( no pun intended )
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There is no such thing as negative matter.\n\nYou may be referring to 1 or 3 things:\n\nAnti-matter: Anti-matter is the "anti" particle to various particles. You have the antiproton, antineutron, positron, etc etc. They are basically particles with opposite charges but similiar other characteristics. \n\nExotic matter: Exotic matter is a hypthetical class of matter that violates current particle physics. Such matter hypothetically can have negative mass, have negative kinetic energies while in motion, or even be repelled by gravity. Some 'wormhole' theories rely on such particles.\n\nDark matter: Dark matter is matter of unknown properties that is being theorized to solve many problems associated with our current understanding of the universe. Such mass does not emit or re-absorb light.
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are you asking about "dark matter"?  dark matter is an unknown kind of matter that only interacts with ordinary matter gravitationally.  about 85 percent of the matter in the universe is dark matter.  swiss astronomer fritz zwicky originated the term 'dark matter'.\n\nread these pages.  they will help.\nhttp://en.wikipedia.org/wiki/Dark_matter\nhttp://map.gsfc.nasa.gov/m_uni.html
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Ask your friend if he knows about "dark matter". \n\nAlmost all the astronomers and most of the physiscists believe nowadays that only 4% of the Universe is made of "normal matter", that is made of neutrons, protons, electrons, photosn, etc (all the aprticles contained in the standard model of Particle Physics). The rest of the Universe is believed to be made of something whose nature is still not known, 26% of this "something" is called dark matter, the name is because it doesn't interact via electromagnetism, therefore, it can not produce, or be affected by light of any frecuency, it is then dark in the sense that no instrument based on light can detect it. The presence of dark matter however affects massive bodies by its gravitational influence, in the last decades its existence has been inferred in objects of very large size, like galaxies and cluster of galaxies, there are many indirect observations that sustain the existence of dark matter, the most stronger is probably the observed rotation curves fo galaxies, which doesn't match the expectations if we account only the visible ("normal") matter.\n\nCandidates for being the components of dark matter are beyond the Standard Model of Particles, going for supersymmetry to more exotic stuff like branes and extra dimmensions. It is a very interesting fields and many projects are currently being carried pout to discover it directly or know more about its nature indirectly.\n\nThe remaininf 70% of the Universe is even a bigger mistery, it is called "dark energy". It differs of dark amtter because contrary to it, it doesn't produce an atraction like the gravitational force, but a repulsion in the large scale Universe, so it is not the same as dark matter, and of course, not the same as normal matter, it is something else, there are theories that try to describe it, but we are still in the beggining of discovering what it is.
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I think that simple answer is, no one knows. We have theories, but we really do not know. \n\nFor example, maybe the antimatter particles decay much faster (see reference). Maybe the universe is only locally mainly matter, and there are other patches of the universe that are locally mainly antimatter (see reference). \n\nWhat science is, is a series of problems and questions that scientists attempt to answer. You have asked one of those questions that is so far, in the unresolved category.
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the most abundant substance in the earths crust is silicon dioxide (sand) and the most abundant on the planet is iron (the core is essentially all iron)
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There currently is no explanation for dark matter just some guesses based on observations.  Some of the observations are the following: \n\n1.  Gravitational lensing from massive galaxies is stronger than expected based upon the average "visible" mass of many galaixes.\n\n2.  Galactic rotation curves do not follow Keplerian motion at distances far from the center of spiral galaxies which seem to imply that there is unseen matter in a halo around most galaxies.  It has been recently proposed that cetain general relativistic treatments of mass distributions of galaxies could explain rotation curves with out the use of dark matter.  It appears that this latter theory is beginning to gain acceptance (see paper under sources).   \n\nIf dark matter does exist then it is matter that doesn't interact (or weakly) via photons (the electromagnetic force), likely not made of electons, but does interact gravitiational.  Nothing currently is known to have these properties.  Objects like brown dwarfs, black holes and white dwarfs were believed to contribute to dark matter however it has been shown that the space density of these object don't add up to a signifcant amount.  \n\nOther theories include MACHOs (Massive Astrophysical Compact Halo Objects; origin unknown) or the theoretical particles called axions and WIMPs (Weakly Interacting Massive Particles).
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Stephen Hawking addressed this issue in his famous book " A Brief History of Time" .\n\nHe stated that if the universe continued to expand the density of matter will continue to decrease with a resulting drop in the average temperature of the universe.\n\nHe feels that eventually, ( as in billions of years ), the only things left in such a large and cold universe will be black holes which will "gobbled up" nearly every atom of matter and energy so that it will be a very dark place.\n\nin fact the real endpoint of the universe is unknown, and it may eventually contract as well if there is enough gravity to pull it back together !