Dark Matter
Dark Matter Constitutes 80% of the Matter in the Universe



Dark Matter - The Hunt for Dark Matter in the Universe
Dark Matter Constitutes 80% of the Matter in the Universe

For the first time ever, astronomers have been creating a three-dimensional map of how the dark matter is distributed across the Universe.

An international team of scientists, among them groups from Marseille, the Max-Planck Institutes and Paris have been using data from the NASA/ESA Hubble Space Telescope.


The results are published in nature online of January 8th 2007, and at the meeting of the American Astronomical Society in Seattle. This Video News Release discussed this discovery.


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 media.


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, while the ordinary matter accounts for only 4.6% (the remainder is attributed to dark energy).

From these figures, dark matter constitutes 80% of the matter in the universe, while ordinary matter makes up only 20%.

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, including 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 remainder is frequently called the "dark matter component," even though there is a small amount of baryonic dark matter. 


The largest part of dark matter, which does not interact with electromagnetic radiation, is not only "dark" but also, by definition, utterly transparent.

The vast majority of the dark matter in the universe is believed to be nonbaryonic, which means that it contains no atoms and does not interact with ordinary matter via electromagnetic forces.

The nonbaryonic dark matter includes neutrinos, and possibly hypothetical entities such as axions, or supersymmetric particles.


Unlike baryonic dark matter, nonbaryonic dark matter does not contribute to the formation of the elements in the early universe ("big bang nucleosynthesis") and so its presence is revealed only via its gravitational attraction.

In addition, if the particles of which it is composed are supersymmetric, they can undergo annihilation interactions with themselves resulting in observable by-products such as photons and neutrinos ("indirect detection").

 
Dark Matter is matter that emits little or no detectable radiation. Gravitational forces observed on many astronomical objects suggest the significant presence of such matter in the universe, accounting for approximately 23 percent of the total mass and energy of the universe.

Its exact nature is not well understood, but it may be largely composed of varieties of subatomic particles that have not yet been discovered, as well as the mass of black holes and of stars too dim to observe. Also called missing mass.




Most Of Our Universe Is Missing

There was a time, not so long ago, when science seemed to understand how the universe worked. Everything us, the Earth, the stars and even exotic-sounding supernovae was made of atoms which were all created at time-zero: the Big Bang.

In between the atoms was nothing, a void: quite literally, 'space'. But recently things have started to unravel. There is, it seems, a lot more to the universe than meets the eye.

According to the best estimates, we only really know what about 4% of it is made of. But if only 4% is made of atoms, what about the rest?

The rest is made of mysterious entities about which very little is understood, with equally mysterious names: dark matter and dark energy.


The Universe - Dark Matter
Hypothesized Form of Matter Particle that does not
Reflect or Emit Electromagnetic Radiation

Nonbaryonic dark matter is classified in terms of the mass of the particle(s) that is assumed to make it up, and/or the typical velocity dispersion of those particles (since more massive particles move more slowly).

There are three prominent hypotheses on nonbaryonic dark matter, called Hot Dark Matter (HDM), Warm Dark Matter (WDM), and Cold Dark Matter (CDM); some combination of these is also possible.


The most widely discussed models for nonbaryonic dark matter are based on the Cold Dark Matter hypothesis, and the corresponding particle is most commonly assumed to be a neutralino. Hot dark matter might consist of (massive) neutrinos.

Cold dark matter would lead to a "bottom-up" formation of structure in the universe while hot dark matter would result in a "top-down" formation scenario.

Dark Matter


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.

As important as dark matter is believed to be in the universe, 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 theories such as modified Newtonian dynamics and tensor-vector-scalar gravity have been proposed.

None of these alternatives, however, has garnered equally widespread support in the scientific community.

If the dark matter within our galaxy is made up of Weakly Interacting Massive Particles (WIMPs), then a large number must pass through the Earth each second. There are many experiments currently running, or planned, aiming to test this hypothesis by searching for WIMPs.

Although WIMPs are a more popular dark matter candidate, there are also experiments searching for other particle candidates such as axions.

It is also possible that dark matter consists of very heavy hidden sector particles which only interact with ordinary matter via gravity.


These experiments can be divided into two classes: direct detection experiments, which search for the scattering of dark matter particles off atomic nuclei within a detector; and indirect detection, which look for the products of WIMP annihilations.

An alternative approach to the detection of WIMPs in nature is to produce them in the laboratory. Experiments with the Large Hadron Collider (LHC) may be able to detect WIMPs; because a WIMP has negligible interactions with matter, it may be detected indirectly as (large amounts of) missing energy and momentum which escape the LHC detectors, provided all the other (non-negligible) collision products are detected.

These experiments could show that WIMPs can be created, but it would still require a direct detection experiment to show that they exist in sufficient numbers in the galaxy, to account for dark matter.



The Missing Universe

What are 'dark matter' and 'dark energy'?

The content of the Universe is widely thought to consist of three types of substance: normal matter, dark matter and dark energy.

Normal matter consists of the atoms that make up stars, planets, human beings and every other visible object in the Universe.

As humbling as it sounds, normal matter almost certainly accounts for the smallest proportion of the Universe, somewhere between 1% and 10%.

The more astronomers observed the Universe, the more matter they needed to find to explain it all. This matter could not be made of normal atoms, however, otherwise there would be more stars and galaxies to be seen. Instead, they coined the term 'dark matter' for this peculiar substance precisely because it escapes our detection.

At the same time, physicists trying to further the understanding of the forces of nature were starting to believe that new and exotic particles of matter must be abundant in the Universe.

These would hardly ever interact with normal matter and many now believe that these particles are the dark matter. At the present time, even though many experiments are underway to detect dark matter particles, none have been successful. Nevertheless, astronomers still believe that somewhere between 30% and 99% of the Universe may consist of dark matter.

Dark energy is the latest addition to the contents of the Universe. Originally, Albert Einstein introduced the idea of an all-pervading 'cosmic energy'; before he knew that the Universe is expanding. The expanding Universe did not need a 'cosmological constant' as Einstein had called his energy.

However, in the 1990s observations of exploding stars in the distant Universe suggested that the Universe was not just expanding but accelerating as well. The only way to explain this was to reintroduce Einstein's cosmic energy in a slightly altered form, called 'dark energy'. No one knows what the dark energy might be.

In the currently popular 'concordance model' of the Universe, 70% of the cosmos is thought to be dark energy, 25% dark matter and 5% normal matter.