Dark Matter should not to be confused with Dark Energy, Dark Fluid (is an alternative theory to both Dark Matter and Dark Energy and attempts to explain both phenomena in a single framework), or Dark Flow (astrophysical term describing a peculiar velocity of galaxy clusters).
In astronomy and cosmology, dark matter is matter that is inferred to exist from gravitational effects on visible matter and background radiation, but is undetectable by emitted or scattered electromagnetic radiation.
Its existence was hypothesized to account for discrepancies between measurements of the mass of galaxies, clusters of galaxies and the entire universe made through dynamical and general relativistic means, and measurements based on the mass of the visible "luminous" matter these objects contain: stars and the gas and dust of the interstellar and intergalactic medium. It is probably cold and if so, probably comprised of weakly interacting massive particles or many primordial intermediate mass black holes between 30 and 300,000 solar masses, or both.
According to observations of structures larger than galaxies, as well as Big Bang cosmology interpreted under the Friedmann equations and the FLRW metric, dark matter accounts for 23% of the mass-energy density of the observable universe. In comparison, ordinary matter accounts for only 4.6% of the mass-energy density of the observable universe, with the remainder being attributable to dark energy. From these figures, dark matter constitutes 83% of the matter in the universe, while ordinary matter makes up only 17%.
Dark matter was postulated by Fritz Zwicky in 1934 to account for evidence of "missing mass" in the orbital velocities of galaxies in clusters. Subsequently, other observations have indicated the presence of dark matter in the universe; these observations include the rotational speeds of galaxies, gravitational lensing of background objects by galaxy clusters such as the Bullet Cluster, and the temperature distribution of hot gas in galaxies and clusters of galaxies.
Dark matter plays a central role in state-of-the-art modeling of structure formation and galaxy evolution, and has measurable effects on the anisotropies observed in the cosmic microwave background. All these lines of evidence suggest that galaxies, clusters of galaxies, and the universe as a whole contain far more matter than that which interacts with electromagnetic radiation. The largest part of dark matter, which does not interact with electromagnetic radiation, is not only "dark" but also, by definition, utterly transparent.
As important as dark matter is believed to be in the cosmos, direct evidence of its existence and a concrete understanding of its nature have remained elusive. Though the theory of dark matter remains the most widely accepted theory to explain the anomalies in observed galactic rotation, some alternative theoretical approaches have been developed which broadly fall into the categories of modified gravitational laws, and quantum gravitational laws.
At present, the density of ordinary baryons and radiation in the universe is estimated to be about one hydrogen atom per cubic meter of space. However, dark matter and dark energy are together said to account for 96% of all matter in the universe. This means that only about 4% of all matter can be directly observed.
Galactic rotation curves, which illustrate the velocity of rotation versus the distance from the galactic center, cannot be explained by only the visible matter. Assuming that the visible material makes up only a small part of the cluster is the most straightforward way of accounting for this. Galaxies show signs of being composed largely of a roughly spherical halo of dark matter with the visible matter concentrated in a disc at the center. Low surface brightness dwarf galaxies are important sources of information for studying dark matter, as they have an uncommonly low ratio of visible matter to dark matter, and have few bright stars at the center which impair observations of the rotation curve of outlying stars.
Recently, astronomers from Cardiff University claim to have discovered a galaxy made almost entirely of dark matter, 50 million light years away in the Virgo Cluster, which was named VIRGOHI21. Unusually, VIRGOHI21 does not appear to contain any visible stars: it was seen with radio frequency observations of hydrogen. Based on rotation profiles, the scientists estimate that this object contains approximately 1000 times as much dark matter as hydrogen and has a total mass of about 1/10th of that of the Milky Way Galaxy we live in. For comparison, the Milky Way is believed to have roughly 10 times as much dark matter as ordinary matter. Models of the Big Bang and structure formation have suggested that such dark galaxies should be very common in the universe, but none have previously been detected. If the existence of this dark galaxy is confirmed, it provides strong evidence for the theory of galaxy formation and pose problems for alternative explanations of dark matter.
Dark matter is believed to affect galaxy clusters as well. The galaxy cluster Abell 2029 is composed of thousands of galaxies enveloped in a cloud of hot gas, and an amount of dark matter equivalent to more than a hundred trillion Suns. At the center of this cluster is an enormous, elliptically shaped galaxy that is thought to have been formed from the mergers of many smaller galaxies.
Large computer simulations of billions of dark matter particles have been used to confirm that the cold dark matter model of structure formation is consistent with the structures observed in the universe through galaxy surveys, such as the Sloan Digital Sky Survey and 2dF Galaxy Redshift Survey, as well as observations of the Lyman-alpha forest. These studies have been crucial in constructing the Lambda-CDM model which measures the cosmological parameters, including the fraction of the universe made up of baryons and dark matter.
Another important tool for future dark matter observations is gravitational lensing, in particular a technique called weak lensing that allows astrophysicists to characterize the distribution of dark matter by statistical means.
Hot dark matter consists of particles that travel with relativistic velocities. One kind of hot dark matter is known, the neutrino. Neutrinos have a very small mass, do not interact via either the electromagnetic or the strong nuclear force and so are incredibly difficult to detect. This is what makes them appealing as dark matter. However, bounds on neutrinos indicate that ordinary neutrinos make only a small contribution to the density dark matter.
Hot dark matter cannot explain how individual galaxies formed from the Big Bang. The microwave background radiation as measured by the COBE and WMAP satellites, while incredibly smooth, indicates that matter has clumped on very small scales. Fast moving particles, however, cannot clump together on such small scales and, in fact, suppress the clumping of other matter. Hot dark matter, while it certainly exists in our universe in the form of neutrinos, is therefore only part of the story.
To explain structure in the universe it is necessary to invoke cold (non-relativistic) dark matter. Large masses, like galaxy-sized black holes can be ruled out on the basis of gravitational lensing data. Possibilities involving normal baryonic matter include brown dwarfs or perhaps small, dense chunks of heavy elements; such objects are known as massive compact halo objects, or "MACHOs". However, studies of big bang nucleosynthesis have convinced most scientists that baryonic matter such as MACHOs cannot be more than a small fraction of the total dark matter.
At present, the most common view is that dark matter is made of one or more elementary particles other than the usual electrons, protons, and neutrons. Currently, the most commonly considered particles are neutrinos, axions, SIMPs (Strongly Interacting Massive Particles), and WIMPs (Weakly Interacting Massive Particles). None of these are part of the standard model of particle physics. Instead, particles in this last category are frequently suggested by theorists proposing supersymmetric extensions of the standard model of particle physics. In such theories, the WIMP involved is usually the neutralino. Another candidate is so-called sterile neutrinos. Sterile neutrinos can be added to the standard model to explain the small neutrino mass. These sterile neutrinos are expected to be heavier than the ordinary neutrinos, and are a candidate for dark matter.
Since it cannot be directly detected via optical means, many aspects of dark matter remain speculative. The DAMA/NaI experiment has claimed to directly detect dark matter passing through the Earth, though most scientists remain skeptical since negative results of other experiments are (almost) incompatible with the DAMA results if dark matter consists of neutralinos.
An alternative to dark matter is to suppose that the inconsistencies are due to an incomplete understanding of gravitation. To explain the observations, the gravitational force has to become stronger than the Newtonian approximation at great distance. For instance, this can be done by assuming a negative value of the cosmological constant (the value of which is believed to be positive based on recent observations) or by assuming Modified Newtonian Dynamics (MOND), which corrects Newton's laws at small acceleration. However, constructing a relativistic MOND theory has been troublesome, and it is not clear how the theory can be reconciled with gravitational lensing measurements of the deflection of light around galaxies. The leading relativistic MOND theory, proposed by Milgrom's colleague Professor Bekenstein in 2004 is called "TeVeS" for Tensor-Vector-Scalar and solves many of the problems of earlier attempts.
Dark Matter Wikipedia
Dark matter images reveal widest view of dark mystery BBC - January 10, 2012
Researchers have released the biggest images yet detailing dark matter, the mysterious substance that makes up 85% of the Universe's mass. Each image, a billion light-years across, shows vast dark matter clumps and voids scattered through the cosmos.
Astronomers create largest map yet of dark matter's web MSNBC - January 9, 2012
Astronomers have created a vast cosmic map revealing an intricate web of dark matter and galaxies spanning a distance of 1 billion light-years. This unprecedented task was achieved not by observing dark matter directly, but by observing its gravitational effects on ancient light traveling from galaxies that existed when the universe was half the age it is now.
Could dark matter not matter? PhysOrg - December 5, 2011
You probably want to put on your skeptical goggles and set them to maximum for this one. An Italian mathematician has come up with some complex formulae that can, with remarkable similarity, mimic the rotation curves of spiral galaxies without the need for dark matter.
Dark-Matter Galaxy Detected: Hidden Dwarf Lurks Nearby? National Geographic - January 15, 2011
Signs point to an invisible "Galaxy X" just outside our own. An entire galaxy may be lurking, unseen, just outside our own. The invisibility of "Galaxy X"- as the purported body has been dubbed - may be due less to its apparent status as a dwarf galaxy than to its murky location and its overwhelming amount of dark matter, astronomer Sukanya Chakrabarti speculates. Detectable only by the effects of its gravitational pull, dark matter is an invisible material that scientists think makes up more than 80 percent of the mass in the universe.
Dark matter 'beach ball' unveiled BBC - January 6, 2010
Milky Way's halo more squished than spherical MSNBC - January 6, 2010
Dark matter's flattened appearance around galaxy surprises scientists.
Dark Matter Detected for First Time? National Geographic - December 18, 2009
The first glimpse of dark matter? BBC - December 18, 2009
Dark Matter And Dark Energy Make Up 95 Percent Of Universe, Detailed Measurements Reveal Science Daily - November 4, 2009
Dark matter 'bridge to nowhere' found in cosmic void New Scientist - September 16, 2008
Cluster Smashup Is Dark Matter Proof National Geographic - August 27, 2008
Bulk of Missing "Normal" Matter Found in Cosmic Web National Geographic - May 21, 2008
Giant ropes of dark matter found in new sky survey New Scientist - February 22, 2008
Huge filaments of dark matter have been detected in a survey of thousands of distant galaxies. The discovery supports the idea that dark matter drove the formation of galaxies and larger cosmic structures and resolves a discrepancy in previous studies about how much dark matter the universe contains.
Astronomers discover largest-ever dark matter structures spanning 270M light-years PhysOrg - February 21, 2008
Violent Lives Of Galaxies: Dark Matter Found Tugging At Galaxies In Supercluster Space Daily - January 15, 2008
Mapping dark matter Guardian - January 11, 2008
Dark Matter May Have Powered Universe's First Stars National Geographic - December 6, 2007
According to a new theory, disintegrating fragments of the mysterious substance could have created "dark stars" hundreds of thousands of times wider than the sun around 13 billion years ago, just after the big bang.
Small 'Hobbit' Galaxies Made Almost Entirely of Dark Matter Live Science - September 12, 2007
Hubble spots ring of dark matter BBC - May 16, 2007
Astronomers have found one of the best pieces of evidence for the existence of dark matter, a mysterious quantity that pervades our Universe.
They have identified what appears to be a ghostly ring in the sky which is made up of this enigmatic substance. Using the Hubble Space Telescope, the scientists have established that the ring formed long ago after a colossal smash-up between two galaxy clusters.
Mapping The Invisible: Dark Matter Charted Out To Five Billion Light Years Science Daily - April 23, 2007
Most of the matter in the Universe is not the ordinary kind made up of protons, neutrons, and electrons, but an elusive "dark matter" detectable only from its gravity. Like a tenuous gas, dark matter is all around us - it goes through us all the time without us noticing - but tends to collect in large quantities around galaxies and clusters of galaxies and makes up about one-sixth of the mass of the Universe.
Hubble makes 3D dark matter map BBC - January 8, 2007
Astronomers have mapped the cosmic "scaffold" of dark matter upon which stars and galaxies are assembled. Dark matter does not reflect or emit detectable light, yet it accounts for most of the mass in the Universe.
Dark Matter: Mysterious has always existed BBC - November 16, 2006
The findings are consistent with the idea of dark energy behaving like Albert Einstein's cosmological constant. The cosmological constant describes the idea that there is a density and pressure associated with "empty" space. In this scenario, dark energy never changes; it has the same properties across the age of the Universe. Einstein first conceived of the notion of a repulsive force in space in his attempt to balance the Universe against the inward pull of its own gravity, which he thought would ultimately cause the Universe to implode. His cosmological constant remained a curious hypothesis until 1998, when astronomers used observations of supernovae from ground-based telescopes and Hubble to show that the expansion of space was accelerating. These findings suggested there really was a repulsive form of gravity in space, a force that was shortly dubbed "dark energy". There have been many attempts to explain the nature of dark energy.
Team finds 'proof' of dark matter BBC - August 21, 2006
US astronomers say they have found the first direct
evidence for the mysterious stuff called dark matter.
Dark matter comes out of the cold BBC - February 5, 2006
Astronomers have for the first time put some real
numbers on the physical characteristics of dark matter.
Dark Matter: Invisible, Mysterious and Perhaps Nonexistent Space.com - October 11, 2005
Galaxies don't have enough regular matter to keep them from flying apart, scientists have been telling us for years. So there must be a bunch of unseen "dark matter" lurking in every galaxy. But dark matter has never been directly detected, and nobody knows what it might be made of.
Supernovae Survey Provides New Clues To Nature Of Mysterious Dark Energy September 2003 - Science Daily
Measurements of 11 exploding stars spread throughout the visible universe made by the Hubble Space Telescope confirm earlier, ground-based studies which produced the first evidence that the universe is not only expanding, but expanding at an increasing rate.
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