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Cosmological models in which dark matter consists of cold elementary particles predict that the dark halo population should extend to masses many orders of magnitude below those at which galaxies can form 1 – 3 . Here we report a cosmological simulation of the formation of present-day haloes over the full range of observed halo masses (20 orders of magnitude) when dark matter is assumed to be in the form of weakly interacting massive particles of mass approximately 100 gigaelectronvolts. The simulation has a full dynamic range of 30 orders of magnitude in mass and resolves the internal structure of hundreds of Earth-mass haloes in as much detail as it does for hundreds of rich galaxy clusters. We find that halo density profiles are universal over the entire mass range and are well described by simple two-parameter fitting formulae 4 , 5 . Halo mass and concentration are tightly related in a way that depends on cosmology and on the nature of the dark matter. For a fixed mass, the concentration is independent of the local environment for haloes less massive than those of typical galaxies. Haloes over the mass range of 10 −3 to 10 11 solar masses contribute about equally (per logarithmic interval) to the luminosity produced by dark matter annihilation, which we find to be smaller than all previous estimates by factors ranging up to one thousand 3 . Simulations of formation of dark matter haloes ranging in size from Earth mass to clusters of galaxies find a universal halo density structure spanning 20 orders of magnitude in mass.

There is a growing sense of ‘crisis’ in the dark-matter particle community, which arises from the absence of evidence for the most popular candidates for dark-matter particles—such as weakly interacting massive particles, axions and sterile neutrinos—despite the enormous effort that has gone into searching for these particles. Here we discuss what we have learned about the nature of dark matter from past experiments and the implications for planned dark-matter searches in the next decade. We argue that diversifying the experimental effort and incorporating astronomical surveys and gravitational-wave observations is our best hope of making progress on the dark-matter problem. The current state of the search for dark-matter particles is reviewed, and a broader experimental and theoretical approach is proposed to solve the dark-matter problem.

The gravitational wave detectors have shown a population of massive black holes that do not resemble those observed in the Milky Way 1 – 3 and whose origin is debated 4 – 6 . According to a possible explanation, these black holes may have formed from density fluctuations in the early Universe (primordial black holes) 7 – 9 , and they should comprise several to 100% of dark matter to explain the observed black hole merger rates 10 – 12 . If these black holes existed in the Milky Way dark matter halo, they would cause long-timescale gravitational microlensing events lasting years 13 . The previous experiments were not sufficiently sensitive to such events 14 – 17 . Here we present the results of the search for long-timescale microlensing events among the light curves of nearly 80 million stars located in the Large Magellanic Cloud that were monitored for 20 years by the Optical Gravitational Lensing Experiment survey 18 . We did not find any events with timescales longer than 1 year, whereas all shorter events detected may be explained by known stellar populations. We find that compact objects in the mass range from 1.8 × 10 −4 M ⊙ to 6.3 M ⊙ cannot make up more than 1% of dark matter, and those in the mass range from 1.3 × 10 −5 M ⊙ to 860  M ⊙ cannot make up more than 10% of dark matter. Thus, primordial black holes in this mass range cannot simultaneously explain a substantial fraction of dark matter and gravitational wave events. The results of the search for long-timescale microlensing events among the light curves of nearly 80 million stars located in the Large Magellanic Cloud indicate that there are no massive black holes in the Milky Way halo.

The astrophysics of the cosmic dawn, when star formation commenced in the first collapsed objects, is predicted to be revealed by spectral and spatial signatures in the cosmic radio background at long wavelengths. The sky-averaged redshifted 21 cm absorption line of neutral hydrogen is a probe of the cosmic dawn. The line profile is determined by the evolving thermal state of the gas, radiation background, Lyman α radiation from stars scattering off cold primordial gas, and relative populations of the hyperfine spin levels in neutral hydrogen atoms. We report a radiometer measurement of the spectrum of the radio sky in the 55–85 MHz band, which shows that the profile found by Bowman et al. in data taken with the Experiment to Detect the Global Epoch of Reionization Signature (EDGES) low-band instrument is not of astrophysical origin; their best-fitting profile is rejected with 95.3% confidence. The profile was interpreted to be a signature of the cosmic dawn; however, its amplitude was substantially higher than that predicted by standard cosmological models. Our non-detection bears out earlier concerns and suggests that the profile found by Bowman et al. is not evidence for new astrophysics or non-standard cosmology. The EDGES team reported a measurement of the redshifted 21 cm absorption line of neutral hydrogen from the cosmic dawn. However, the SARAS 3 measurement of the radio sky spectrum now suggests that the EDGES detection might not have a cosmological source.

The nature of dark matter remains unknown to date, although several candidate particles are being considered in a dynamically changing research landscape 1 . Scalar field dark matter is a prominent option that is being explored with precision instruments, such as atomic clocks and optical cavities 2 – 8 . Here we describe a direct search for scalar field dark matter using a gravitational-wave detector, which operates beyond the quantum shot-noise limit. We set new upper limits on the coupling constants of scalar field dark matter as a function of its mass, by excluding the presence of signals that would be produced through the direct coupling of this dark matter to the beam splitter of the GEO600 interferometer. These constraints improve on bounds from previous direct searches by more than six orders of magnitude and are, in some cases, more stringent than limits obtained in tests of the equivalence principle by up to four orders of magnitude. Our work demonstrates that scalar field dark matter can be investigated or constrained with direct searches using gravitational-wave detectors and highlights the potential of quantum-enhanced interferometry for dark matter detection. Using a gravitational-wave detector to  listen for dark matter signatures, a direct search for scalar field dark matter was conducted and new upper limits are set on the coupling constants.

The ultra-diffuse galaxies DF2 and DF4 in the NGC 1052 group share several unusual properties: they both have large sizes 1 , rich populations of overluminous and large globular clusters 2 – 6 , and very low velocity dispersions that indicate little or no dark matter 7 – 10 . It has been suggested that these galaxies were formed in the aftermath of high-velocity collisions of gas-rich galaxies 11 – 13 , events that resemble the collision that created the bullet cluster 14 but on much smaller scales. The gas separates from the dark matter in the collision and subsequent star formation leads to the formation of one or more dark-matter-free galaxies 12 . Here we show that the present-day line-of-sight distances and radial velocities of DF2 and DF4 are consistent with their joint formation in the aftermath of a single bullet-dwarf collision, around eight billion years ago. Moreover, we find that DF2 and DF4 are part of an apparent linear substructure of seven to eleven large, low-luminosity objects. We propose that these all originated in the same event, forming a trail of dark-matter-free galaxies that is roughly more than two megaparsecs long and angled 7° ± 2° from the line of sight. We also tentatively identify the highly dark-matter-dominated remnants of the two progenitor galaxies that are expected 11 at the leading edges of the trail. The dark-matter-free dwarf galaxies DF2 and DF4 in the NGC 1052 group probably formed together in the aftermath of a single bullet-dwarf collision around eight billion years ago.

The concordance model (Λ cold dark matter (ΛCDM) model, where Λ is the cosmological constant) reproduces the main current cosmological observations1–4 assuming the validity of general relativity at all scales and epochs and the presence of CDM and of Λ, equivalent to dark energy with a constant density in space and time. However, the ΛCDM model is poorly tested in the redshift interval between the farthest observed type Ia supernovae5 and the cosmic microwave background. We present measurements of the expansion rate of the Universe based on a Hubble diagram of quasars. Quasars are the most luminous persistent sources in the Universe, observed up to redshifts of z ≈ 7.5 (refs. 6,7). We estimate their distances following a method developed by our group8–10, based on the X-ray and ultraviolet emission of the quasars. The distance modulus/redshift relation of quasars at z < 1.4 is in agreement with that of supernovae and with the concordance model. However, a deviation from the ΛCDM model emerges at higher redshift, with a statistical significance of ~4σ. If an evolution of the dark energy equation of state is allowed, the data suggest dark energy density increasing with time.The concordance cosmology model is poorly tested at high redshifts. Here the expansion rate of the Universe in the range 0.5 < z < 5.1 is measured based on a Hubble diagram of quasars, whose distances are estimated from their X-ray and ultraviolet emission.

The recent Planck Legacy 2018 release has confirmed the presence of an enhanced lensing amplitude in cosmic microwave background power spectra compared with that predicted in the standard Λ cold dark matter model, where Λ is the cosmological constant. A closed Universe can provide a physical explanation for this effect, with the Planck cosmic microwave background spectra now preferring a positive curvature at more than the 99% confidence level. Here, we further investigate the evidence for a closed Universe from Planck, showing that positive curvature naturally explains the anomalous lensing amplitude, and demonstrating that it also removes a well-known tension in the Planck dataset concerning the values of cosmological parameters derived at different angular scales. We show that since the Planck power spectra prefer a closed Universe, discordances higher than generally estimated arise for most of the local cosmological observables, including baryon acoustic oscillations. The assumption of a flat Universe could therefore mask a cosmological crisis where disparate observed properties of the Universe appear to be mutually inconsistent. Future measurements are needed to clarify whether the observed discordances are due to undetected systematics, or to new physics or simply are a statistical fluctuation. The standard cosmological model assumes a flat Universe, but some model inconsistencies appear when curvature is allowed, as supported by the latest Planck Legacy 2018 power spectra. Is it time to consider new physics?

Ultralight axion-like particles are well-motivated relics that might compose the cosmological dark matter and source anomalous time-dependent magnetic fields. We report on terrestrial bounds from the Noble And Alkali Spin Detectors for Ultralight Coherent darK matter (NASDUCK) collaboration on the coupling of axion-like particles to neutrons and protons. The detector uses nuclei of noble-gas and alkali-metal atoms and operates in the Spin-Exchange Relaxation-Free (SERF) regime, achieving high sensitivity to axion-like dark matter fields. Conducting a month-long search, we cover the mass range of 1.4 × 10 −12 eV/ c 2 to 2 × 10 −10 eV/ c 2 and provide limits which supersede robust astrophysical bounds, and improve upon previous terrestrial constraints by over two orders of magnitude for many masses within this range for protons, and up to two orders of magnitude for neutrons. These are the sole reliable terrestrial bounds reported on the coupling of protons with axion-like dark matter, covering an unexplored terrain in its parameter space. Axions are hypothetical particles that constitute leading candidates for the identity of dark matter. Here, the authors improve previous exclusion bounds on axion-like particles in the range of 1.4–200 peV, and report direct terrestrial limits on the coupling of protons and neutrons with axion-like dark matter.

Unveiling the true nature of dark matter, which manifests itself only through gravity, is one of the principal quests in physics. Leading candidates for dark matter are weakly interacting massive particles or ultralight bosons (axions), at opposite extremes in mass scales, that have been postulated by competing theories to solve deficiencies in the Standard Model of particle physics. Whereas dark matter weakly interacting massive particles behave like discrete particles (ϱDM), quantum interference between dark matter axions is manifested as waves (ψDM). Here, we show that gravitational lensing leaves signatures in multiply lensed images of background galaxies that reveal whether the foreground lensing galaxy inhabits a ϱDM or ψDM halo. Whereas ϱDM lens models leave well documented anomalies between the predicted and observed brightnesses and positions of multiply lensed images, ψDM lens models correctly predict the level of anomalies remaining with ϱDM lens models. More challengingly, when subjected to a battery of tests for reproducing the quadruply lensed triplet images in the system HS 0810+2554, ψDM is able to reproduce all aspects of this system whereas ϱDM often fails. The ability of ψDM to resolve lensing anomalies even in demanding cases such as HS 0810+2554, together with its success in reproducing other astrophysical observations, tilt the balance toward new physics invoking axions.Modelling of the gravitationally lensed system HS 0810+2554 with wavelike dark matter resolves brightness and position anomalies remaining after the standard massive-particle dark matter treatment.