Detection of dark matter

These cosmological models predict that if WIMPs are what make up dark matter, trillions must pass through the Earth each second. Despite a number of attempts to find these WIMPs, none have yet been found.

Experimental searches for these dark matter candidates have been conducted and are ongoing. These efforts can be divided into two broad classes: direct detection, in which the dark matter particles are observed in a detector; and indirect detection, which looks for the products of dark matter annihilations. Dark matter detection experiments have ruled out some WIMP and axion models. There are also several experiments claiming positive evidence for dark matter detection, such as DAMA/NaI, DAMA/LIBRA and EGRET, but these are so far unconfirmed and difficult to reconcile with the negative results of other experiments. Several searches for dark matter are currently underway, including the Cryogenic Dark Matter Search in the Soudan mine, the PICASSO experiment in the SNOLAB underground laboratory at Sudbury, Ontario (Canada), XENON, DAMA/LIBRA and CRESST experiments at Gran Sasso (Italy) and the ZEPLIN and DRIFT projects at the Boulby Underground Laboratory (UK), and many new technologies are under development, such as the ArDM or MIMAC experiments.

One possible alternative approach to the detection of WIMPs in nature is to produce them in the laboratory. Experiments with the Large Hadron Collider near Geneva may be able to detect the WIMPs. Because a WIMP only has negligible interactions with matter, it can be detected as missing energy and momentum. It is also possible that dark matter consists of very heavy hidden sector particles which only interact with ordinary matter via gravity.

The Cryogenic Dark Matter Search, in the Soudan Mine in Minnesota aims to detect the heat generated when ultracold germanium and silicon crystals are struck by a WIMP. The Gran Sasso National Laboratory at L'Aquila, in Italy, use xenon to measure the flash of light that occurs on those rare occasions when a WIMP strikes a xenon nucleus. The results from April 2007, using 15 kg of liquid and gaseous xenon, detected several events consistent with backgrounds, setting a new exclusion limit. The larger XENON100 detector, with 150 kg of liquid xenon, began taking calibration data in March 2008.

The PAMELA payload (launched 2006) may find evidence of dark matter annihilation.

The Fermi space telescope, launched June 11, 2008, searching gammawave events, may also detect WIMPs. WIMP supersymmetric particle and antiparticle collisions should release a pair of detectable gamma waves. The number of events detected will show to what extent WIMPs comprise dark matter.

With all these experiments together, scientists are becoming confident that WIMPs will be discovered in the near future. But some scientists are beginning to think that dark matter is composed of many different candidates. WIMPs may thus only be a part of the solution.

In 2014 the lsst will be operational, one of the main goals of the telescope is to discover and learn more about dark matter.

Alternative explanations

Modifications of gravity

A proposed alternative to physical dark matter particles has been to suppose that the observed 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 distances or in weak fields. One of the proposed models is Modified Newtonian Dynamics (MOND), which adjusts 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 Jacob Bekenstein in 2004 is called TeVeS for Tensor-Vector-Scalar and solves many of the problems of earlier attempts. However, a study in August 2006 reported an observation of a pair of colliding galaxy clusters whose behavior, it was claimed, was not compatible with any current modified gravity theories.http://en.wikipedia.org/wiki/Dark_Matter - cite_note-26

In 2007, John W. Moffatt proposed a theory of modified gravity (MOG) based on the Nonsymmetric Gravitational Theory (NGT) that claims to account for the behavior of colliding galaxies.http://en.wikipedia.org/wiki/Dark_Matter - cite_note-27

Quantum mechanical explanations

In another class of theories one attempts to reconcile gravitation with quantum mechanics and obtains corrections to the conventional gravitational interaction. In scalar-tensor theories, scalar fields like the Higgs field couple to the curvature given through the Riemann tensor or its traces. In many of such theories, the scalar field equals the inflaton field, which is needed to explain the inflation of the universe after the Big Bang, as the dominating factor of the quintessence or Dark Energy. Using an approach based on the exact renormalization group, M. Reuter and H. Weyer have shownhttp://en.wikipedia.org/wiki/Dark_Matter - cite_note-28 that Newton's constant and the cosmological constant can be scalar functions on spacetime if one associates renormalization scales to the points of spacetime. Some M-Theory cosmologists also propose that multi-dimensional forces from outside the visible universe have gravitational effects on the visible universe meaning that dark matter is not necessary for a unified theory of cosmology.

Dark matter in popular culture

Mentions of dark matter occur in some video games and other works of fiction. In such cases, it is usually attributed extraordinary physical or magical properties. Such descriptions are often inconsistent with the properties of dark matter proposed in physics and cosmology.