Closing Editorial of Special Issue: The Dark Universe: The Harbinger of a Major Discovery

In the 1930s, Fritz Zwicky [1] was the first to observe an invisible mass in the Coma Cluster, naming it “dunkle Materie,” or “dark matter” (DM), and using it to explain cosmic-scale gravitational puzzles involving galaxies. In the 1970s, Vera Rubin and W. Kent Ford [2] demonstrated that galaxies rotate too fast, implying massive invisible halos. In particle physics, the search for dark matter is an ongoing hunt and has been for several decades, with many candidates emerging as potential DM particles, but all remain hypothetical.

The contributions in this Special Issue present the state of the art of DM research, including both direct and indirect experiments involving both in-orbit and Earth-bound telescopes. They also elucidate a number of new, theoretical concepts, guided by state-of-the-art cosmological reasoning, suggesting novel DM candidates beyond the often-cited axions, WIMPs, and mirror matter particles, etc.

The search for DM constituents has been fruitless thus far, which has driven theorists to develop novel approaches. Here, we are indebted to the exciting axion antiquark nugget (AQN) model proposed by Ariel Zhitnitsky in 2003 [3]. Interestingly, a variety of cosmic observations have been found to fit this model, along with anomalies observed locally, i.e., within our solar system, including the puzzling behavior of the dynamic inner Earth and Earth’s atmosphere. The “planetary dependencies” [4] derived are unexpected within contemporary models of physics as no remote force beyond the extremely feeble gravity is thought to exist. Expanding Zwicky’s reasoning, persisting local mysteries may tentatively be seen as new indications of the dark sector. Noticeably, all solar system bodies are well-performing gravitational lenses for slow DM constituents, with a widely accepted velocity distribution of around 300 km/s. Recall that the gravitational impact proceeds at 1/(speed)². The analysis of several observations within our solar system fits with the streaming DM scenario following the gravitational lensing effects caused by solar system bodies. This Special Issue showcases the ongoing DM research, encompassing emerging experimental, observational, and theoretical ideas (see, e.g., [3]). We start with the axion, one of the leading DM candidates.

Mike Tobar and collaborators [5] introduced an instrument-defined spectral sensitivity for axion haloscopes that enables the fair comparison between the data of dissimilar electromagnetic and acoustic detectors, from broadband concepts to ultra-narrowband high-Q resonators, without baking in specific axion signal assumptions. The key idea is to separate transduction (axion → photon conversion in the experimental geometry and magnetic field) from instrument noise such that the resulting sensitivity curve becomes a portable “figure of merit” across very different architectures and readout chains. The central message, and the reason this work continues to resonate, is that the same resonant electromagnetic infrastructure can also function as a high-frequency gravitational wave (HFGW) antenna. In a background magnetic field, gravitational radiation can be converted to electromagnetic excitations (i.e., via inverse Gertsenshtein-type processes), allowing one to recast resonant sensors in a GW-style strain spectral sensitivity framework. This has helped to clarify how “axion hardware” can be repurposed (or co-designed) to access a complementary GW window, and it directly aligns with the rapid emergence of GravNet, an ERC-funded effort aiming to build a network of synchronized cavity-based sensors in magnetic fields to push into the MHz–GHz GW band. In parallel, the broader program now clearly spans electromagnetic and acoustic resonant antennae: the multi-mode acoustic gravitational-wave experiment (MAGE) has demonstrated multi-band, narrow-linewidth strain sensitivity using cryogenic quartz bulk-acoustic resonators in the MHz regime, providing a concrete benchmark for resonant mass HFGW sensing and a valuable point of comparison for cavity-based approaches. Together, GravNet-style cavity networks and MAGE-style acoustic resonators underline the same strategic direction: a common spectral sensitivity language is fast becoming the facilitating tool for rationally combining and optimizing axion and gravitational antennae across disparate platforms.

The next-generation axion haloscope (DALI) [6] occupies a distinctive position in the axion DM landscape as its new magnetized phased-array architecture reaches QCD-inspired sensitivity using only existing technology. Reassessing its performance through Monte Carlo simulations with synthetic data reveals that its discovery range extends over a broad axion mass range above 25 μeV.

One of the new theoretical notions is that DM could be realized in the form of the axion quark nuggets (AQN) made of standard-model quarks (or antiquarks) and gluons [3], similar to the old idea of Witten’s strangelets. This concept implies dramatic changes for our standard 40-year-old paradigm, from weakly interacting DM components to strongly interacting objects in the form of AQNs. This proposal is an alternative to the commonly accepted scenario wherein the “baryogenesis” is replaced by a charge-separation process in which the global baryon number of the universe always remains zero. This represents the key element of the AQN construction. In the AQN scenario, the DM density, Ω_DM, representing matter and anti-matter nuggets, as well as the visible density, Ω_visible, will automatically—irrespective of the axion mass, m_a, or the misalignment angle, θ₀—assume the same order of magnitude densities, i.e., Ω_DM~Ω_visible, as they are both proportional to the same fundamental dimensional parameter of the theory, the Λ_QCD. Therefore, the AQN model, by construction, resolves two fundamental problems in cosmology without the necessity of fitting any of the model parameters; that is, it explains the baryon asymmetry of the universe and the presence of DM with proper density (Ω_DM~Ω_visible). Furthermore, the same AQNs might simultaneously explain many of the mysterious excess emissions observed at all scales, from the early universe to the galaxy, the solar system, and the Earth.

More specifically, AQNs may also explain some of the long-standing local puzzles apparent in the ionosphere, the stratosphere, and the inner Earth, including anomalies observed in cross-disciplinary measurements [4,7]. Interestingly, the AQN DM model, when applied at the microscopic level [7,8], quantitatively accounts for puzzling observations in both the atmosphere and the lithosphere through realistic energy deposition in the dynamic Earth’s interior and atmosphere. Importantly, the analysis follows the DM framework based on the cosmological DM-to-baryon ratio (Ω_DM~Ω_visible) rather than on phenomenological tuning, thereby providing testable manifestations of strongly interacting DM and elevating anomalies from empirical curiosities to direct, testable manifestations of strongly interacting DM. Beyond the large energies involved in earthquakes, so far, the largest energy involvement is associated with the rhythmic solar size variation over a period of several years. Remarkably, it also shows planetary dependency [9], which indicate the presence of streaming invisible matter, with the strongly interacting AQNs being one possible cause, among other cosmic constituents such as magnetic monopoles, dark photons, or other objects which have not yet been proposed. Nevertheless, AQNs provide a concrete and predictive explanation for long-standing cosmic and local (i.e., atmospheric and geophysical) anomalies that have remained unexplained for decades. The direct detection of AQNs or something similar is highly promising.

New methods have recently been proposed for the direct detection of AQNs. One method is based on data already gather by the ~4000 detectors along the LHC cryo-ring via the expected intense irradiation around 100–200 keV photons [10].

Another novel idea for the direct detection of AQNs is the use of natural minerals as paleo-detectors [11]. The latent signals of (thermo)luminescence produced by the interactions of AQNs are stored for geological time scales and can be identified as an increased deposited dose.

Furthermore, it has been proposed that an AQN hitting the Earth can be detected by analyzing infrasound, acoustic, and seismic waves, which should accompany the passage of AQNs in the atmosphere and underground [8]. A new concept has been proposed for the interaction of AQNs with matter, involving the detection of relatively large DM nuggets. It has been suggested that the detection of the estimated infrasonic frequency (ν≃5 Hz) and overpressure (δp∼0.3 Pa) for relatively large DM nuggets is within the reach of existing technology.

Other DM candidates and modes of DM searching have also been proposed. This Special Issue includes a number novel theoretical/experimental ideas concerning the discovery of DM. Some examples are as follows:

• PBHs: Cosmological primordial black holes (PBHs), if they have survived to the present epoch, have been proposed to comprise a major portion of the DM in the cosmos [12]. In addition, if a PBH has evaporated before the present epoch, rare forms of DM such as super weakly interacting or supermassive particles could have been produced during the evaporation. If stable, they could be DM candidates, as well as a major physical sign of the working of the dark sector. Thus, the conventional cosmic rays may contain overlooked exotic DM constituents, suggesting that we search for such DM particles beyond the widely discussed axions and WIMPs.
• Dark atoms: Hypothetical “dark atoms” consist of stable lepton-like particles that remain undiscovered in experiments due to their neutral configuration [13]. Numerical modeling is employed to unravel the interactions of “dark atoms” with nuclei.
• n-Decay: Neutron decay has provided important insight into the weak nuclear force [14]; it is also important for understanding the formation and abundance of light elements in the early universe, with implications for cosmology, astrophysics, and DM. Surprisingly, the two measuring methods of a neutron’s lifetime differ by 9.8 ± 2.0 s. If this ~5σ discrepancy is not experimental in origin, it may be explained by new physical phenomena, with possible connections to dark matter. This is a challenging issue, suggesting a truly novel DM concept—neutron coupling.
• Helioseismology: Following helioseismologically measurements, the sun’s size changes by ~10⁻⁵ during its cycle [8]. It has been found that this 1–2 km size variation resembles the 225-day orbital period of Venus, implying a special link between Venus and the sun. This unexpected behavior points towards a low-speed penetrating stream aligned toward the sun and an intervening planet (e.g., Venus, Mercury, Mars, etc.), occasionally increasing the invisible streaming influx due to planetary gravitational focusing. The slow impact accumulates with time, eventually reaching the huge energy deposit necessary to explain this 11-year rhythm. Interestingly, the sun’s size response is half the orbital period of Mercury (44 days) or Venus (112 days), which is quite short for an object like our sun. Thus, the solar system is both the target of, and the antenna for, a still unidentified external impact. It has tentatively been suggested that the dark sector is the cause, or else something that has not yet been discovered.
• Axions introduced to solve the strong CP problem: Remarkably, a new experimental approach (namely, the echo method) for the direct detection of the celebrated axion dark matter candidate was recently devised by Arza and Sikivie (see [15]). If radio/microwave radiation is sent out into space, in the presence of axion dark matter, it is backscattered due to stimulated axion decay. The backscattered photon energy is close to one half of the axion mass. So far, at least three papers have considered this process. Unbiased streaming DM axions fit the underlying scenario due to the built-in strong flux enhancements (up to ~10⁹×) owing to the gravitational focusing effects of solar system bodies, including via their intrinsic mass distribution.

Each novel idea provides a new opportunity to unravel the DM signatures hidden in the existing data, eventually in a model-independent way. Telescopes may have obtained DM-relevant data worth re-analyzing, and the same applies to the data gathered from the plethora of direct DM searches. Streaming DM can occasionally result in planetary dependencies. So far, direct DM searches have anticipated an annual distribution, and the underlying noise usually has a similar seasonal variation. To overcome this inherent noise problem, one may project the measured events (i.e., signal plus noise), for example, on the 88th and/or 225th days of the orbital periodicity of Mercury and Venus, respectively. As of now, the DM axion antiquark nugget model proposed by Zhitnitsky remains a promising favorite.

Our aim is for this Special Issue to function as a source of novel, state-of-the-art, highly sensitive detection concepts for physicists interested in pursuing the recent developments in the field of dark matter research.