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.

Dark Matter Wikipedia

Giant Dark Matter Bridge Between Galaxy Clusters Discovered   Live Science - July 5, 2012
A giant string of invisible dark matter has been discovered across the universe between a pair of galaxy clusters. The filament forms a bridge between two huge clusters called Abell 222 and Abell 223, which lie 2.7 billion light-years away. The universe is thought to be filled with such strings of dark matter, a mysterious substance that cannot be seen, only sensed through its gravitational pull. Scientists have made previous attempts to find dark matter filaments, which are predicted by theories that suggest galaxy clusters form at the intersections of filaments. Dark matter is thought to make up 98 percent of all matter in the universe.

Dark Matter Hits the Average Human Once a Minute?   National Geographic - April 24, 2012
The average human body gets hit by a particle of dark matter about once a minute, according to new calculations based on several dark matter detection efforts. Dark matter is an invisible form of material that's thought to exist because scientists have observed its apparent gravitational effects on galaxies and galaxy clusters. Scientists estimate that the mysterious substance makes up almost 80 percent of the matter in the universe. So far no one's been able to pinpoint the particles that make up dark matter. But a leading candidate is a theoretical group known as Weakly Interacting Massive Particles, or WIMPs.

Dark Matter Is Missing in Sun's Neighborhood?   National Geographic - April 19, 2012
Dark matter is mysteriously missing from the sun's neighborhood, according to a new study that could provide ammunition for skeptics who argue that the invisible substance is just an illusion. Even though dark matter particles can't be detected with current instruments, astronomers think the substance must make up about a quarter of the universe, based on the gravitational effect it has on visible matter such as galaxies and galaxy clusters. Since its discovery in the 1930s, the material has become crucial for galaxy-formation theories, which say that dark matter functions as a kind of invisible scaffold around which normal matter gravitationally coalesces to form stars and larger objects.

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.

The first glimpse of dark matter?   BBC - December 18, 2009
US scientists have reported the detection of signals that could indicate the presence of dark matter. A team announced on Thursday detecting two events with characteristics "consistent with" what physicists believe make up the elusive matter. The main announcement came from the Department of Energy's Fermi National Accelerator Laboratory near Chicago. The scientists were keen to stress that they could not confirm that what they had seen was definitely dark matter.

Dark Matter And Dark Energy Make Up 95 Percent Of Universe, Detailed Measurements Reveal   Science Daily - November 4, 2009
A detailed picture of the seeds of structures in the universe has been unveiled by an international team co-led by Sarah Church of the Kavli Institute for Particle Astrophysics and Cosmology, jointly located at the Department of Energy's SLAC National Accelerator Laboratory and Stanford University, and by Walter Gear, of Cardiff University in the United Kingdom. These measurements of the cosmic microwave background -- a faintly glowing relic of the hot, dense, young universe -- put limits on proposed alternatives to the standard model of cosmology and provide further support for the standard cosmological model, confirming that dark matter and dark energy make up 95% of everything in existence, while ordinary matter makes up just 5%.

Cluster Smashup Is Dark Matter Proof National Geographic - August 27, 2008
If the latest image from the Hubble Space Telescope looks familiar, that's because this candy-hued galaxy cluster could be a twin of the so-called bullet cluster, a formation hailed in 2006 as the first direct proof for the existence of dark matter. At 5.6 billion light-years from Earth, the new cluster known as MACS J0025.4-1222 - is much farther away, and thus older, than its famous relative. But the formation shows the same separation of dark and ordinary matter occurring as its two parent galaxy clusters collide at high speed.

Bulk of Missing "Normal" Matter Found in Cosmic Web National Geographic - May 21, 2008

Much of the missing "normal" matter in the cosmos has been found clustered around wispy ropes of invisible matter spanning the space between galaxies. The filaments form part of the vast weblike superstructure of the universe, within which galaxies are embedded like sparkling sequins.

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

Dark energy - the mysterious force that is speeding up the expansion of the Universe - has been a part of space for at least nine billion years. Dark energy makes up about 70% of the Universe; the rest is dark matter (25%) and normal matter (5%). "It appears this dark energy was already boosting the expansion of the Universe as much as nine billion years ago," said co-investigator Adam Riess from the Space Telescope Science Institute in Baltimore, US. "That's out of a Universe which we think is about 13.7 billion years old - most of the way back."

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.