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The mystery of dark matter

In this image, dark matter (blue) has become separated from luminous matter (red) in the bullet cluster. (Image courtesy: Chandra)

There's more of the universe that we don't understand than we do understand. Ordinary matter—the stuff that scientists have spent decades studying—makes up around five percent of the universe. The remainder is thought to be comprised of dark energy (around 70 percent) and dark matter (around 25 percent). What is all this dark stuff and how do we know it's there if we can't even see it directly?

We know that dark matter exists because it acts on the cosmos in a number of ways. In the 1930s, an astrophysicist named Fritz Zwicky realized that, in order to act the way they do, galaxy clusters must contain a lot more mass than was actually visible. If the galaxies also contained unseen "dark" matter, everything made a lot more sense. Then, in the 1970s, astronomer Vera Rubin discovered that stars at the edge of a galaxy move just as quickly as stars near the center. This observation makes sense if the visible stars were surrounded by a halo of something invisible: dark matter. Since then, a number of other astronomical observations have confirmed the effects of dark matter.

Several dozen experiments are now on the hunt for stronger evidence that dark matter exists. Many of these experiments look for Weakly Interacting Massive Particles, or WIMPs. Others search for a particle called the axion, a theoretical neutral particle that interacts with other particles extraordinarily weakly, or theorized dark-matter versions of the photon.

Experiments generally hunt for dark-matter particles in two ways: either through a direct search in which dark-matter particles bump into target material and scatter off atomic nuclei, resulting in a measurable nuclear recoil (these experiments are usually located underground, where there's little background noise), or through an indirect search for particles that should appear if a dark matter particle annihilates (these experiments are generally conducted with ground-based or space satellite telescopes). It is also thought that if dark matter particles can annihilate into regular (or Standard Model) particles, then the reverse could be true, and that dark matter particles could be created during high-energy collisions like those at the Large Hadron Collider.

More information on recent dark matter experiments and results can also be found in this compilation of recent dark matter news.

Dark matter experiments: direct detection

Annual modulation with NAI Scintillators (ANAIS)

ANAIS was developed by the Nuclear Physics and Astroparticles group of the University of Zaragoza in Spain. The experiment pursues dark matter by looking at the annual modulation of the expected interaction rates in a target of sodium iodide (NaI), material which produces small scintillations when a particle interacts and deposits some energy. This modulation is a distinctive feature stemming from the Earth's revolution around the sun, which changes periodically the relative velocity of incoming dark matter particles to the detector and, because of that, the energy deposited. DAMA-LIBRA experiment at Gran Sasso Underground Laboratory has reported the presence of modulation in its data with a high statistical significance; ANAIS could confirm it and help to understand the different systematics involved.

Argon Dark Matter experiment (ArDM)

ArDM seeks to detect weakly interacting massive particles (WIMPs). It looks for nuclear recoils produced by dark matter particles scattering off target nuclei within 1‑ton of liquid argon. The experiment has been tested aboveground at CERN, and was recently moved to the Laboratorio Subterráneo de Canfranc in northern Spain.

Axion Dark Matter Experiment (ADMX)

ADMX, located at the University of Washington, searches for the axion, a hypothetical neutral elementary particle that has only extraordinarily feeble interactions with normal matter and radiation. Light axions are thought to have been copiously produced in the early universe and may constitute dark matter. The ADMX experimental technique is to thread a tunable microwave cavity with a large static magnetic field. Nearby halo axions would thereby convert into microwave photons, and those photons would be detected by near-quantum-limited receivers. This direct-detection, laboratory-based experiment is the only dark-matter axion search sensitive enough to detect plausible dark-matter axions with the expected masses and interaction strengths.

Chicagoland Observatory for Underground Particle Physics (COUPP)

COUPP-60, located at the Canadian SNOLAB underground site, searches for weakly interacting massive particles (WIMPs) using stable, room-temperature bubble chambers. The chamber itself is a jar filled with purified water and trifluoromethyl iodide (CF3I), shielded in 7,000 gallons of water. When a dark matter particle enters the detector and hits a nucleus in a trifluoromethyl iodide molecule, it should trigger the evaporation of a small amount of the liquid, causing the formation of a bubble. The bubble grows and eventually becomes large enough for researchers to see it, at which point they photograph the chamber with digital cameras. Sensitive microphones also help distinguish between nuclear recoils and alpha particles. PICASSO and COUPP have now merged to form the PICO collaboration.

CDEX (China Dark matter Experiment), PANDA-X (Particle AND Astroparticle with Xenon), and TEXONO (Taiwan EXperiment On NeutrinO) Experiments

The China Jin-Ping Underground Laboratory, in the middle of a 18-km tunnel under 2400 meters of rock, houses both the CDEX and PANDA-X experiments. The CDEX Collaboration is led by Beijing Tsinghua University and searches for weakly interacting massive particles (WIMPs) in the low mass region (less than 10 gigaelectronvolts or GeV) using point-contact germanium semi-conductor detectors. PANDA-X, led by Shanghai Jiaotong University, is a direct-detection experiment using a two-phase (liquid and gas) xenon detector to search for WIMPs. The TEXONO research program, initiated in 1997, is based at Academia Sinica, Taiwan. The TEXONO Collaboration performs neutrino and dark matter experiments at the Kuo-Sheng Reactor Neutrino Laboratory in Taiwan, and participates in the CDEX experiment in the China Jin-Ping Underground Laboratory since 2009.


The CoGeNT experiment looks for a type of dark-matter particle called a WIMP, or weakly interacting massive particle, specifically those relatively light in mass. Recent data obtained at the Soudan Underground Laboratory present an excess of events that might be compatible with WIMPs in the mass range of 7‑11 billion electronvolts, and a low-significance annual modulation in the same energy region. CoGeNT continues to take data at Soudan, and a next generation of the CoGeNT experiment, intended to deploy four larger-mass detectors, is now underway.

Cryogenic Dark Matter Search (CDMS/SuperCDMS)

The CDMS collaboration uses solid-state detectors cooled to extremely low temperatures to search for weakly interacting massive particles (WIMPs), a favored candidate for dark matter. Operating initially in a shallow underground site at Stanford University (1996‑2002) and then in Minnesota's Soudan Mine (2003‑2009), CDMS detectors have produced world-leading sensitivity to WIMPs. A few hints of possible signals have been found, but have since been eliminated by more recent data from CDMS and other experiments. With larger and more-sensitive SuperCDMS detectors, the experiment now continues the search for WIMPs at Soudan (2011‑2015) and is approved for a new experiment for the deeper, cleaner underground site at SNOLAB in Ontario, Canada. Preliminary planning is also underway for the Germanium Observatory for Dark Matter (GEODM), a third-phase upgrade with an even larger detector mass.

Cryogenic Rare Event Search with Superconducting Thermometers (CRESST)

CRESST searches for weakly interacting massive particles, or WIMPs, using detectors operated at extremely low temperatures, where WIMP interactions can heat up the detectors strong enough in order to be detected. The first CRESST WIMP limits were achieved in 2004. In 2011, CRESST observed an excess of events above the background expectation in the dark matter region of interest. The same dataset could exclude inelastic dark matter as the explanation for the signal in DAMA/LIBRA under standard assumptions. The experiment, located in the Gran Sasso underground laboratory in Italy, is currently (2013‑2014) undergoing upgrades that will improve the CRESST detectors and shielding, making the experiment even more sensitive to evidence of dark matter.


In 1998, scientists on the DAMA experiment, a dark matter detector buried in Italy’s Gran Sasso mountain, saw a promising pattern in their data. The rate at which the experiment detected hits from possible dark-matter particles changed over the course of the year—climbing to its peak in June and dipping to its nadir in December. This is what would be expected if our galaxy is surrounded by a dark matter halo; during half of the year, the Earth is moving in the same direction as the sun, and their combined velocity through the dark matter halo is faster than the Earth’s velocity when it and the sun are at odds. DAMA’s results seemed to reveal that the Earth really was moving through a dark matter halo. The DAMA experiment, now called DAMA/LIBRA, is constructed of thallium-doped sodium iodide (NaI(Tl)) crystals, and continues to observe this annual modulation.

Dark Matter WIMP Search in Noble Liquids (DARWIN)

The goal of DARWIN is to complete the necessary research for the construction of the ultimate dark matter detector, using several tons of liquid xenon and/or liquid argon for the direct detection of particle dark matter. DARWIN aims at pushing the sensitivity to new levels in order to eventually cover the entire WIMP-parameter space before neutrino backgrounds dominate. Such a detector would not only have a realistic chance of discovering the nature of dark matter, but would also be able to study its properties such as mass, interaction strength and its local distribution in our galaxy. For the liquid xenon detector, other physics channels are in reach as well.

Dark Matter Search Experiment with Liquid Argon Pulse Shape Discrimination (DEAP)

DEAP 3600 is a 3,600kg liquid Argon single phase detector that will start taking physics data in 2015. It is located at SNOLAB in Sudbury, Ontario, roughly 2 kilometers underground. As weakly interacting massive particles (WIMPs) travel through the detector, they will elastically scatter off argon nuclei and cause light to be emitted. The detector will be sensitive to dark matter interactions as low as to 10-46 cm2 per nucleon for a WIMP mass of 100 GeV. A 7-kilogram liquid-argon dark matter detector, DEAP-1 has been used as a prototype and test-bed for DEAP-3600 and its sister experiment MiniCLEAN.


The DarkSide program proposes to develop and operate a series of novel liquid argon detectors for the detection of weakly interacting massive particles. The detectors of the DarkSide program will use several innovative techniques to positively identify dark matter signals and to understand and suppress backgrounds. These techniques include the use of argon from underground rather than atmospheric sources, an active neutron veto to strongly suppress neutron backgrounds; and comprehensive measures to control background sources in the detector and photosensors. The first phase is DarkSide-50 (DS-50 Time Projection Chamber), with 50 kilograms active mass, which has been operating beneath 1,400 meters of rock in the underground INFN Gran Sasso Laboratories, located in Abruzzo, Italy, since autumn 2013. DS-50’s background suppression strategy uses 30 tons of liquid scintillator for neutron detection, and 1,000 tons of water to detect muons, both of which can mimic the signal of a dark matter particle.

Experiment for Direct Detection of WIMP Dark Matter (EDELWEISS)

The EDELWEISS experiment is dedicated to the direct detection of dark matter particles from within France’s Modane Underground Laboratory. The first two stages of the EDELWEISS experiment obtained excellent sensitivities—placing the experiment as one of the most sensitive in the world—but did not observe any weakly interacting massive particles (WIMPs). The third stage of the experiment, EDELWEISS-III was installed in spring 2014, and is now taking data.

European Underground Rare Event Calorimeter Array (EURECA)

EURECA is a planned experiment that would seek to detect the scattering of weakly interacting massive particles (WIMPs) by atomic nuclei, using cryogenic detectors operating at millikelvin temperatures. EURECA is designed to follow the EDELWEISS and CRESST experiments, which will run through 2015. Design work for EURECA is currently underway, and operation is expected to begin sometime after 2017. EURECA will be built in the Laboratoire Souterrain de Modane in France.

Finding U(1)s of a Novel Kind (FUNK)

One of the prominet candidates for light-weight dark matter particles are called hidden photons. Closely related to real photons, they should have weak electromagnetic interactions and should be able to excite electrons in electrical conductors. The experiment FUNK (Finding U(1)s of a Novel Kind, with U(1) being a technical term for photons) in Karlsruhe, Germany, measures photonic signals in the center of a large spheric metallic mirror as a hint to the existence of hidden photon dark matter. When hidden photons interact with the metal of the mirror, a real photon with a wavelength corresponding to the very small mass of the hidden photon should be emitted. A receiver in the focus of the roughly 13-square-metres mirror is to detect these emerging photons.

Korea Invisible Mass Search (KIMS)

KIMS searches for weakly interacting massive particles (WIMPs) from within the preexisting Yangyang Pumped Storage Power Plant in Yangyang, Korea. There, several large, pure pure cesium iodide (CsI) crystals would light up with the detection of WIMPs. In addition to searching for dark matter, the collaboration is also developing detectors for a neutrinoless double beta decay search and designing an extreme low temperature diamond calorimeter.

Large Hadron Collider (LHC)

If dark matter particles are light enough, they may be produced at the LHC at CERN. If they were created at the LHC, they would escape through the detectors unnoticed. However, they would carry away energy and momentum, so physicists could infer their existence from the amount of energy and momentum “missing” after a collision. More than 100 experimental physicists are part of this search for identify dark matter candidates.


The LZ experiment is a planned second-generation dark matter experiment at South Dakota's Sanford Underground Research Facility. LZ will look for signals caused by the collisions of weakly interacting massive particles (WIMPs) with xenon atoms in seven metric tons of liquid xenon. The LZ apparatus will be located inside the 72,000-gallon water tank now occupied by the Large Underground Xenon (LUX) experiment 4850 feet below ground. LZ brings together LUX with the smaller Zoned Proportional scintillation in Liquid Noble (ZEPLIN) series of experiments, which ran until 2011 in the Boubly Underground Laboratory in the United Kingdom.

Project In Canada to Search for Supersymmtric Objects (PICASSO)

The PICASSO project is a dark matter search experiment presently installed and taking data in the SNOLAB underground laboratory at Sudbury, Ontario, Canada. PICASSO’s detectors are made up of millions of tiny droplets of a liquid that’s “superheated,” or heated well above its boiling point. Such a liquid is extremely unstable, and any slight perturbation—such as the passage of a weakly interacting massive particle (WIMP)—will trigger an explosive transformation of the liquid into vapor. This mini-explosion would be accompanied by an acoustic pulse, which can be picked up by acoustic sensors. PICASSO's detector is tuned to see the neutralino, which is a neutral hypothetical particle more than 100x heavier than the proton that interacts very weakly with ordinary matter. PICASSO and COUPP have now merged to form the PICO collaboration.

Superheated Instrument for Massive Particle Experiments (SIMPLE)

The SIMPLE experiment searches for direct evidence of dark matter. From deep with in the Laboratoire Souterrain à Bas Bruit in southern France, the experiment’s detectors use superheated chloropentafluoroethane (C2CIF5) to detect the passage of a weakly interacting massive particle (WIMP).


The XENON Dark Matter Search, underway in Italy’s Laboratori Nazionali del Gran Sasso, uses liquid xenon in its hunt for dark matter particles. Through a combination of large mass and ultra-low background detectors, from XENON10 to XENON100, and in the near future XENON1T, the XENON project has led the field for years with some of the most stringent limits on the interaction between weakly interactive massive particles (WIMPs) and normal matter.


The XMASS project aims to detect dark matter, "pp" and "7Be" solar neutrinos, and neutrinoless double beta decay using ultra pure liquid xenon. The liquid xenon detector XMASS-I, dedicated to search for dark matter particles, was constructed in Japan's Kamioka Observatory. A total of 642 inward-pointing photomultiplier tubes detect xenon scintillation light. XMASS-I's copper vessel is placed at the center of a cylindrical water tank 10 meters in diameter and 11 meters in height to shield it from external background noise. Physics data-taking began in November 2013.

Dark matter experiments: indirect detection

Alpha Magnetic Spectrometer (AMS)

AMS is a precision particle physics detector installed on the International Space Station. It is a US-DOE led international collaboration involving 56 institutes from 16 countries. The detector measures particles (electrons, positrons, antiprotons) and nuclei to the TeV energy range. 51 billion events have been collected and 11 million positron-electron events have been analyzed. The positron data show that the spectrum cannot be explained by collisions of ordinary cosmic rays. Collisions of dark matter would produce an excess of positrons. This possibility is being studied. AMS will be on the Space Station for at least 10 more years and will provide a sensitive search for dark matter in space.

Any Light Particle Search (ALPS I and ALPS II)

ALPS is a laser experiment, based at the DESY laboratory in Germany, that searches for photon oscillations into WISPs (very weakly interacting sub-electronvolt particles). One of the most well-known WISP candidates is the axion, also a candidate for dark matter. ALPS shines a laser into a magnetic field to convert some of the photons into WISPs. The laser light is then blocked by a barrier, while WISPs can traverse the barrier and convert back to detectable photons on the other side.

Astronomy with a Neutrino Telescope and Abyss environmental RESearch (ANTARES)

ANTARES is an underwater neutrino telescope located 2475 meters under the Mediterranean Sea. Weakly interacting massive particles (WIMPs), one of the preferred candidates for dark matter, could theoretically self-annihilate in the center of massive astrophysical objects such as the sun. This annihilation would create Standard Model particles that then decay into high-energy neutrinos. ANTARES is also very sensitive to WIMPs accumulated in the centre of our galaxy, since it is highly visible from the ANTARES location. ANTARES uses an array of photomultiplier tubes in vertical strings, spread over an area of about 0.1 square kilometers to search for statistical excesses of neutrinos coming from these sources.

BAIKAL Neutrino Telescope

The BAIKAL Neutrino Telescope NT-200 is located in the Siberian lake Baikal at a depth of approximately 1 kilometer. Its Cherenkov detector consists of 192 optical sensors deployed on eight strings and was constructed to study high-energy muon and neutrino fluxes in cosmic rays. It measures dark matter in the form of neutrinos created from dark matter annihilation in the center of the sun.

The Cherenkov Telescope Array (CTA)

CTA is a planned next-generation, ground-based, very-high-energy gamma-ray instrument. Theoretical models predict that dark matter can annihilate or decay to detectable Standard Model particles, including a large number of very-high-energy gamma rays. With its two telescope arrays (one in the northern hemisphere and one in the southern hemisphere), CTA will search for these gamma rays with unprecedented sensitivity, providing a tool to study the properties of dark matter particles. If candidate dark matter particles are discovered at the Large Hadron Collider or in underground experiments, CTA will aim to verify whether they actually form the dark matter in the universe.

Fermi Gamma-ray Space Telescope-Large Area Telescope (FGST-LAT)

The Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope (FGST) works to unveil the high-energy universe. Through its space-based studies of gamma rays, FGST-LAT reveals insight into everything from supernova remnants to active galactic nuclei. High-energy gamma rays are expected to stream from dark matter, making FGST-LAT a powerful dark matter hunter. Two of the most easily interpretable signals would come from dwarf galaxies that are extremely rich in dark matter, and a sharp excess in signal at a specific gamma-ray energy coming from the entire galaxy. Since the beginning of the mission, a number of papers have been published by the LAT collaboration improving dark matter limits; these measurements will dramatically improve as the mission continues. There are also hints of a dark matter signal from the center of the Milky Way from non-LAT collaboration authors. These hints are controversial, and new results are expected from the LAT collaboration by mid-2014.

General Antiparticle Spectrometer (GAPS)

GAPS is a proposed balloon-based indirect dark matter search focusing on antiparticles produced by WIMP (weakly interacting massive particle) annihilation and decay in the Galactic halo. Anti-deuterons are thought to be a very promising way to search for dark matter, as a dark matter produced signal would have very low background from conventional (cosmic-ray) production at low kinetic energies. Unlike conventional magnetic spectrometers, GAPS uses the exotic-atom technique, making it complementary to the standard approach. The GAPS detector will use layers of lithium-doped silicon—Si(Li)—detectors surrounded by time of flight plastic scintillators. The time of flight system serves as an anti-coincidence shield and records particle velocity. The particle then slows down and stops in the Si(Li) target and goes through a series of interactions until it annihilates with the emission of pions and protons which are measured. Characteristic X-rays are also emitted and detected in the Si(Li) detectors. A prototype flight (pGAPS) was successfully conducted in 2012. A first GAPS science flight is proposed for 2018‑2019.

Heavy Photon Search (HPS)

HPS will start hunting for new particles that mediate dark matter interactions, known as dark matter force particles, at Virginia's Jefferson Lab in late 2014. In this direct, laboratory-based search for dark-matter photons, HPS will send electrons streaming into a sheet of tungsten foil and look for electron-positron pairs resulting from the decay of a heavy photon. If these particle pairs appear some distance from the foil, that will indicate that an unseen particle—like the dark-matter photon—was created first, then flew a certain distance before decaying into the electron-positron pair.

High Energy Stereoscopic System (HESS)

The HESS observatory uses a system of Imaging Atmospheric Cherenkov Telescopes, located in Namibia, to investigate very high-energy cosmic gamma rays. The experiment looks for the predicted gamma-ray annihilation signal from weakly interacting massive particles (WIMPs) in various regions considered to have enhanced dark matter density, including the galactic center, Sagittarius dwarf, M87, and clumps in the galactic halo. HESS is operated by the collaboration of more than 170 scientists, from 32 scientific institutions and 12 different countries.

IceCube Neutrino Observatory

The IceCube Neutrino Observatory is a particle detector at the South Pole that records the interactions of a nearly massless subatomic particle called the neutrino. IceCube searches for neutrinos from the most violent astrophysical sources: events like exploding stars, gamma ray bursts, and cataclysmic phenomena involving black holes and neutron stars. Dark matter may gravitationally cluster (or, for the sun and Earth, be gravitationally captured) and build up. As the density rises, it begins to self-annihilate. IceCube is the world's largest neutrino detector, encompassing a cubic kilometer of ice.

The Isotope Matter Antimatter Experiment (IMAX)

IMAX is a balloon-borne, superconducting magnet spectrometer experiment designed to measure the galactic cosmic ray abundances of protons, antiprotons, deuterium, helium-3, and helium-4. The IMAX collaboration includes NASA Goddard Space Flight Center, Caltech Space Radiation Lab, the University of Siegen, New Mexico State University, and the Danish Space Research Institute. The University of Arizona participated with a piggy-backed dark matter experiment that searched for ionizing massive particles (IMPs) that cannot penetrate the atmosphere due to their low-velocities and high energy-loss.

The Major Atmospheric Gamma-ray Imaging Cherenkov Telescopes (MAGIC)

The MAGIC telescopes consist of two ground-based, Imaging Atmospheric Cherenkov Telescopes located at the Roque de Los Muchachos on Spain’s Canary Island of La Palma. The telescopes include nearly 1,000 individual mirrors, together resulting in a parabolic dish with a 17-meter diameter, and a camera that detects images of gamma-ray induced air showers in blueish Cherenkov light. The telescopes measure gamma-ray sources in the very high-energy range 30 GeV – 50 TeV. These rays may result from the annihilation of weakly interacting massive particles (WIMPs), which are major candidates for dark matter.

Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA)

PAMELA is a cosmic ray research detector onboard the Resurs-DK1 Russian satellite that searches for the annihilation products of weakly interacting massive particles (WIMPs). WIMPs are favoured candidates of dark matter. PAMELA indirectly searches for the existence of dark matter by looking for antiprotons and positrons that may result from the annihilations of WIMPs.

Super-Kamiokande (SK)

SK is a 50 kilo-ton water Cherenkov detector which started operation in 1996. The detector is located in the Kamioka Observatory, 1,000 meters beneath Mt. Ikenoyama in Japan's Gifu Prefecture. SK is dedicated to searching for nucleon decays and observing neutrinos from various sources. An indirect dark matter search is being performed by SK by looking for neutrinos and neutrino-induced muons from annihilations of WIMPs in the Earth, the sun, and the galaxy's center and halo. SK has an excellent sensitivity to lower mass weakly interacting massive particles due to its lower neutrino energy threshold.

Very Energetic Radiation Imaging Telescope Array System (VERITAS)

VERITAS is a series of four ground-based imaging Cherenkov telescopes operating at the Fred Lawrence Whipple Observatory in southern Arizona, USA. Each telescope reflector has 350 individual mirrors, a 499 pixel camera with an aperture of 12 metres. VERITAS focuses on the indirect search for very-high-energy gamma rays (with energies of the order of 100 gigaelectronvolts or GeV) from dwarf galaxies which would result from the interaction or decay of dark matter particles.

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