FOR every gram of visible matter in the universe that emits and absorbs light—matter that makes up all the stars and galaxies in the cosmos—there is more than 5 grams of matter out there that is completely invisible or “dark”, stuff that does not interact with any form of electromagnetic radiation at all but interacts only via gravity. That is, unlike electrons and other fundamental particles of nature that interact both through gravity and electric charge, dark matter does not seem to have any other interaction except gravitation. One needed to necessarily hypothesise such mysterious, all-pervasive matter to explain observations made on the motion of stars within galaxies, and that of galaxies within galactic clusters, during the 20th century. The gravitational pull of visible matter alone was not enough to explain, for instance, the rotational velocities around the galactic centre of stars in the distant arms of a spiral galaxy. Scientists reasoned that there must be a large amount of some other kind of matter that cannot be seen but which nevertheless exerts its gravitational pull on celestial bodies.

There are other astrophysical observations as well, to explain which one necessarily needs to invoke dark matter, which coexists with normal matter everywhere: the observed large-scale structure of the visible universe, gravitational lensing (the bending of light from distant galaxies by massive galactic clusters in the foreground to produce multiple images), detailed measurements on the ultra-cold cosmic microwave background radiation that the universe is awash with, and observed abundance of light elements compared with the predictions of the process of nucleosynthesis after the Big Bang that produced them. Furthermore, astronomers believe that they may have already seen direct evidence of dark matter in action in the “bullet cluster” collision that was observed by the Chandra X-ray Observatory (“Direct proof of dark matter?”, Frontline, October 6, 2006).

Also, a team of international researchers, led by Priyamvada Natarajan of Yale University, recently published (in the journal Monthly Notices of the Royal Astronomical Society) one of the highest resolution maps of dark matter, which makes a detailed case for the existence of this mysterious matter. This map was derived from the Hubble Space Telescope data of a trio of galaxy clusters that act as gravitational lenses. “While we now have a precise cosmic inventory for the amount of dark matter and how it is distributed in the universe, the particle itself remains elusive,” Priyamvada Natarajan was quoted as saying. The map, according to the researchers, very closely matches computer simulations of dark matter predicted theoretically by the Standard Model of Cosmology (SMC).

The understanding today, which forms the basis of the SMC, is that the total mass-energy of the universe contains 4.9 per cent of ordinary matter, 26.8 per cent of dark matter and 68.3 per cent of what is called “dark energy” (Figure 1). (“Dark energy”, too, is a hypothetical repulsive force that permeates space and drives the apparent accelerated expansion of the universe, an inference from the observations on Type Ia supernovae made in the 1990s. But, like “dark matter”, scientists have no idea about the form of this energy that is speeding up the expansion of the universe. One of the keys to understanding the universe, therefore, is unravelling this dominant dark side of the universe that makes up nearly 95 per cent of its mass-energy content. The dark energy issue will be discussed in the second part of this two-part article.)

So, we now know that 85 per cent of all mass in the universe is made of “dark matter”. But, from the perspective of particle physics, we have no clue about what its properties are—whether dark matter is one particle or many, and what kind of interactions besides gravitation, if any, it has. Many theoretical candidates for dark matter have been hypothesised by particle physicists. But none of them has shown up in any of the experiments conducted over the last four decades with continually increasing sensitivities that have systematically looked for them and have excluded different possibilities one by one.

Most recently, in 2016, three experiments that were looking for the most compelling and the most promising candidate called WIMPs (Weakly Interacting Massive Particles), one of which was billed as the most sensitive of all dark-matter searches so far, have failed to detect any. But scientists are not giving up as yet. Even as larger detectors with even greater sensitivities are getting ready, experiments to look for a more exotic class of candidates are also on the anvil. The null results obtained so far have prompted theorists to consider a wider canvas of particle models and propose even more exotic possibilities for dark matter. (Alternative cosmological models, which are based on modifications of Einstein’s theory of gravitation and do not require the existence of dark matter, have not been able to explain observations across all spatial and temporal scales, according to the majority of the astronomical community.)

Theoretical candidates



Beside WIMPs, theoretical dark-matter candidates include MAssive Compact Halo Objects (MACHOs), primordial black holes and axions. All the particle candidates in the SMC have to be what is called “cold” dark matter (CDM), as against hot or warm dark matter, where the terms refer not to the temperature of the particles but the velocities with which they were freely streaming owing to random motion in the early universe and how far they would have travelled before being slowed down by the expansion of the universe. CDM particles were slow-moving that became non-relativistic early enough and were no longer freely streaming before density fluctuations of the early universe got washed out. If the density fluctuations were larger than the size of the denser region of slowed-down particles, the imprints of density fluctuations, which are needed to seed clumping of matter, would have remained in such matter and that in turn would have led to the formation of galaxies and other large-scale structures. Particles are classified as cold, warm or hot depending on their respective free streaming lengths as compared to the size of a protogalaxy (a formation in the early universe from which a dwarf galaxy would form later as the universe evolved).

MACHOs is the collective name given to black holes, neutron stars, faint old white dwarfs and brown dwarfs, which are made of normal baryonic matter (matter composed of baryons—protons and neutrons). But, on the basis of several arguments, MACHOs have been nearly ruled out as a dark matter candidate. MACHOs, for instance, would have caused strong gravitational lensing effects that astronomers do not see.

Following the recent detection of gravitational waves by LIGO (Laser Interferometer Gravitational-Wave Observatory), the source of which is inferred to be the merger of two black holes with masses about 30 times that of the sun, primordial black holes in this mass regime had emerged as a possible dark matter candidate. Since the collapse of ordinary stars cannot result in such heavy black holes, it was believed that these must have been produced at the time of the Big Bang itself. And, it was argued, if there were a lot of such heavy black holes left over from the Big Bang (primordial) lurking around, they could be the missing dark matter that is holding the galaxies together and not letting them fly apart. But, very soon, this possibility died as detailed observations of the motion of stars within galaxies revealed that they were not consistent with the predicted influence of 30-solar-mass primordial black holes on stellar motion in galaxies.

Moreover, it is generally believed that dark matter is likely to be non-baryonic, that is, matter that is not composed of protons and neutrons. One of the arguments for that comes from the observed abundance of light elements in the universe (hydrogen, deuterium, helium and lithium) that were formed by nucleosynthesis within the first few minutes of the Big Bang, while heavier elements were formed in the interior of stars which were formed much later.

According to the Big Bang Nucleosynthesis theory, roughly 25 per cent of the mass of the universe consists of helium, about 0.01 per cent deuterium and even smaller quantities of lithium. These numbers depend critically on the density of baryons at the time of nucleosynthesis. Agreement with the observed abundance today requires baryonic matter to constitute around only 4 to 5 per cent of the universe’s critical density. (Critical density is the minimum matter density required to ensure that the universe will not expand forever but will not collapse back on itself either—what is called a “flat” geometry universe.) Against this small value, large-scale structure and other observations indicate that the total matter density is about 30 per cent of the critical density, which implies the presence of dominantly non-baryonic dark matter that would not have contributed to the formation of light elements in the early universe. This forces scientists to consider exotic candidates beyond the particles of the Standard Model of particle physics.

CDM candidates, such as WIMPs and axions, are non-baryonic, of which WIMPs, which are also products of the Big Bang just like particles of normal matter, are the most favoured dark matter candidates. There exists no clear definition of a WIMP, but for it to be a dark matter particle, it should have gravitational interaction and other (known or unknown) force (or forces) with unknown interaction strength(s). Such a particle is not part of the Standard Model framework in particle physics, but a WIMP is required to have a non-gravitational interaction that is weak or weaker than the weak nuclear force of the Standard Model (which causes radioactive decay) with a non-zero interaction strength. But a WIMP can be a particle in the so-called “supersymmetric” extension of the Standard Model, which has been proposed as a solution to resolve some of the shortcomings of the Standard Model, in which each particle of the Standard Model has a “supersymmetric partner”.

After having been produced thermally at the time of the Big Bang, WIMPs should have the right probability of self-annihilation into other particles (called cross section) so that, once created in the Big Bang, we are left with the correct abundance required for WIMPs to fit the dark matter bill. A WIMP particle in the 100 gigaelectronvolt (GeV) mass range (100 times a proton’s mass) that interacts through the electroweak force of the Standard Model is said to have this property; that is, it is stable enough to be around in right numbers even today.

The supersymmetric extension of the Standard Model, which has been proposed to fill the gaps that remain in the Standard Model and which predicts a new “supersymmetric partner” for every particle of the Standard Model, is known to readily accommodate such a particle. The lightest of the supersymmetric particles has the right properties: it is its own right anti-particle so that when two such particles interact they will annihilate, giving rise to photons that can be detected, and it also has the right cross section to be an ideal dark matter candidate. This apparent coincidence is called the “WIMP miracle”, by which the shortcomings of the Standard Model are resolved on the one hand and we have a right candidate to solve the dark matter puzzle on the other. Therefore, such a stable supersymmetric particle has long been posited as a prime WIMP candidate.

Elusive particles



But supersymmetric particles have proved to be highly elusive. If such particles existed, they were expected to show up in the experiments conducted at the Large Hadron Collider (LHC) where very high-energy protons or atomic nuclei smash head-on with each other to recreate the conditions of the early universe. But, so far, even during the recent second run of the LHC with higher collision energy, no supersymmetric particle has been found. So, no WIMPs at the LHC!

The other way to detect dark matter candidates, WIMPs in particular, is by looking for the products of their interactions. Such detection experiments are of two kinds: indirect detection and direct detection. The former looks for the products of their annihilation or decays, such as gamma rays, neutrinos and cosmic rays, in outer space—in nearby galaxies and galaxy clusters. However, such signatures would be extremely difficult to disentangle from a host of astrophysical processes involving normal matter that would produce similar particles. As a result, experiments that have been primed to detect these, including the highly sensitive IceCube experiment in Antarctica (“New window to the universe”, Frontline, June 27, 2014), have so far only been able to set some weak bounds on their existence. However, new indirect detection experiments with much better discrimination ability are being considered.

If there is dark matter in the universe in such huge amounts, zillions of these dark matter particles should be passing through the earth every second. The idea of direct detection experiments is to set up detectors where these particles would bump into the nuclei of suitably chosen material and observe the low-energy recoil of nuclei caused by the resulting interaction. As the nuclei recoil, they ionise the matter around, emitting energy in the form of flashes of light, which can be detected (Figure 2). Typically, most such experiments operate deep underground to reduce interference from cosmic rays and background radiation. The overburden of earth above the detectors helps to effectively absorb most of the cosmic ray particles. By looking for the rate at which these flashes occur (a measure of dark matter particle’s interaction strength) and how bright they are (a measure of dark matter particle mass), one can measure the particle’s properties (Figure 3).

Several such experiments are being conducted around the world for direct detection of WIMPs. The Liquid Underground Xenon (LUX) experiment at the Sanford Underground Research Facility, South Dakota, United States, and the Particle and Astrophysical Xenon Detector (PandaX-II) at the China Jin Ping Laboratory in Sichuan, China, are currently the most sensitive of these. These, and the experiment called Xenon100, located in the Gran Sasso mountain in Italy, which use ultra pure liquid xenon for target nuclei, have so far failed to detect any signature for WIMPs. These experiments are said to be most sensitive to particles with masses in the range of 40 to 50 GeV. PandaX-II came out with its results in September 2016 and LUX more recently in January 2017. As a result, the possible parameter space for WIMPs, in terms of WIMP particle mass and interaction strength, is shrinking.

“The lack of any signals rules out a large number of WIMP dark matter candidates, but it is important to remember that the range of interaction strengths and masses of possible dark matter particles is vast,” says Vikram Rentala, a particle physicist at the Indian Institute of Technology Bombay. “As we increase the sensitivity of these experiments, we can look for dark matter particles that interact extremely weakly, but the range of masses that we can probe with this strategy is limited from about 0.01 of the mass of the Higgs boson (~125 GeV) to 10 times the Higgs boson mass,” he adds.

One might ask if there is a limit on how weak the dark matter interaction strength can be. A priori, there is no limit, Rentala points out. “However, if the dark matter interaction strength is extremely low, then neutrinos, which too can reach these underground detectors, start becoming a source of noise in the detector and would completely overwhelm any dark matter signal that we hope to see. Thus, there is a natural limit on the sensitivity that these experiments can hope to reach,” says Rentala. This limit on the dark matter interaction strength is called the “neutrino floor”.

The stakes for dark matter search, as Rentala points out, are high, and the series of null results is becoming a source of worry for the particle physics community. “People are a little nervous,” Leslie Rosenberg, a physicist of the University of Washington in Seattle, was quoted as saying by Nature. “Particle physicists are cautiously optimistic that WIMPs are the right dark matter candidates because they naturally fit into many theories that go beyond the known Standard Model. If it turns out that WIMPs are not the right candidate for dark matter, that would be interesting as well because that would focus our efforts on searches for other dark matter candidates. However, it is far too early to make a claim that WIMPs cannot be the dark matter just because these experiments have not seen it yet,” says Rentala.

Future strategies



One strategy for future dark matter probes is to widen the parameter space of WIMPs. One way is to reduce the mass limit of WIMPs by choosing different detector materials (such as lighter material germanium or silicon instead of xenon). The SuperCDMS (Cryogenic Dark Matter Search) experiment will be doing precisely this by using supercooled germanium and silicon crystalline detector substrates. The other is to increase the sensitivity for even weaker interaction strengths, that is, moving further towards the “neutrino floor” by building bigger detectors to increase recoil rates. This class of experiments includes the next generation experiments like LUX-Zeplin (LZ) and Xenon1000.

Of course, there is also a dark matter detection experiment called DINO, proposed to be located underground at the India-based Neutrino Observatory (INO) to come up in Theni district, Tamil Nadu. However, this experiment stands stalled and inordinately delayed, thanks to misplaced activism and the totally what-do-we-care political attitude in the Tamil Nadu state machinery. “Within 20 years timeline, we should have a definitive answer to whether WIMPs make up dark matter or not,” says Rentala.

The other strategy is to begin to look seriously for other dark matter candidates such as axions. Axions were proposed decades ago to resolve a particle physics quandary called the “strong CP problem”. The question posed by this problem is, why does strong nuclear force, which holds particles together inside the nucleus, not treat matter and antimatter differently as the weak nuclear force does (what is known as CP violation), which plays a role in the dominance of matter over antimatter in the universe. If dark matter was made of axions, it could solve two problems at once. But their masses are extremely low compared with WIMPs—a millionth of a billionth of WIMP mass—which makes them extremely difficult to detect. Axion Dark Matter eXperiment (ADMX), led by Leslie Rosenberg, is aimed at looking for these by using intense magnetic fields and special detection techniques that would convert axions into photons, which can be detected. The principle being, if axions have such tiny mass, trillions of them should be filling every cubic metre of space and intense magnetic fields should be able to convert them into microwave photons.

On the one hand, dark matter may turn out to be completely different from what had been anticipated, disappointing many theorists and experimenters. Particle physicists are already beginning to explore such possibilities in newly fashioned particle theory models. On the other, for someone like Rentala, the “future outlook in the field is terrific”. “There are a number of different experiments looking for WIMPs, and each of them gives us a different piece of the puzzle. A convincing signal at any one experiment could suddenly put several pieces of the jigsaw puzzle in place, and we may well be on the verge of revealing a beautiful picture of nature,” says Rentala.