PUBLISHED: 2 MARCH 2017 | VOLUME: 1 | ARTICLE NUMBER: 0082 editorial Light in darkness We think dark matter exists because measurements of ‘normal’ matter would not otherwise make sense. In this issue — our Insight on dark matter, offered jointly with Nature Physics — we showcase the various techniques trying to make sense of it. Without some kind of ‘missing mass’, galaxy compact halo objects (MACHOs; which 13, 224–231; 2017) bring us up to speed on rotation measurements would have galaxies could be brown dwarfs, black holes or other the current status of indirect dark matter spinning too fast to be stable; therefore objects that emit little or no light). Axions detection. Besides cosmic and gamma rays something must be holding them together. are also a viable candidate. They are neutral, there are other messengers that could tell This something could be an invisible form of very weakly interacting particles that may us something about dark matter: high- matter, called dark matter. The clumpiness explain why there is no charge–parity energy neutrinos. Neutrino telescopes of large-scale galaxy structure would also be symmetry in the strong interaction. What such as IceCube can place limits on WIMP very difficult to explain without such dark is appealing about axions (and other annihilation cross-sections, complementing matter, as would the measured anisotropy of axion-like particles) is the possibility for those determined from direct and collider the cosmic microwave background (CMB). their detection in small-scale experiments. experiments. In his Review Article, Of course, dark matter is not the only Mature axion dark matter searches — such Francis Halzen discusses the present and possible explanation. There are theories as the Axion Dark Matter eXperiment, Any future of high-energy neutrino astronomy that do not invoke any invisible matter. Light Particle Search and CERN Axion (Nat. Phys. 13, 232–238; 2017). Modified Newtonian dynamics (MOND), Solar Telescope, for example — have placed If dark matter holds visible matter in first proposed by Mordehai Milgrom, posits limits on axion–photon coupling. But as we galaxies tighter together, there is another that Newton’s laws do not apply to low- cannot cover all dark matter candidates in dark entity that does the opposite. We acceleration conditions, such as experienced one go, we focus on WIMP searches, which have known since the 1920s that the by stars in the outer regions of galaxies. But have made the most experimental progress. Universe is expanding at a rate determined despite some successes in explaining other WIMPs are a class of stable, neutral by Hubble’s constant, as explained by phenomena, including the Tully–Fisher particles. The most popular are those from Barbara Ryden (Nat. Phys. 13, 314; 2017). relation for a galaxy’s total baryonic mass supersymmetry models that extend the But in the late 1990s, type Ia supernovae and its rotation velocity, there remains standard model, such that each standard measurements revealed that the expansion some missing mass in galaxy clusters within model particle has a supersymmetric rate is accelerating. Assuming the model MOND models, which might well be partner. In one type of experiment, the of Big Bang cosmology is correct, the dark matter. idea is that if these particles exist, Galactic acceleration of the expansion of our So let us examine dark matter. In WIMPs must be passing through Earth, (observationally) flat Universe implies our Insight, we look at how the history and would scatter off target nuclei, for that something must be doing the of dark matter research goes hand in instance within a noble-liquid target. pushing — dark energy — or again we hand with the history of cosmology. Recoiling nuclei would emit light, which need to modify the theory of gravity. But From the 1930s, the evidence for dark can be detected by scintillators. Jianglai Liu, let us hold onto Einstein’s general relativity matter increased to a point when, in the Xun Chen and Xiangdong Ji (Nat. Phys. for now, and consider dark energy, as it 1970s, the cosmology community had 13, 212–216; 2017) discuss the status of is measurable, albeit indirectly. This force to take action. This ground is covered such direct detection experiments, using only interacts with gravity and is invisible by Jaco de Swart, Gianfranco Bertone noble liquids and solid-state detectors. At to electromagnetic radiation; it pushes and Jeroen van Dongen in their Review higher energy scales, particle colliders are visible and dark matter alike. Precision Article (Nat. Astron. 1, 0059; 2017). In smashing particles together and looking measurements of galaxies and galaxy his Perspective, James Peebles recalls for WIMPs in the annihilation or particle clusters reveal the underlying galaxy the convergence of evidence and ideas decay channels. In their Progress Article, structure; the CMB — the geometry of the in the 1980s that made the theory of Oliver Buchmueller, Caterina Doglioni and Universe; and thousands of supernovae — nonbaryonic dark matter compelling Lian-Tao Wang describe efforts at the Large the expansion rate of the Universe. The (Nat. Astron. 1, 0057; 2017). And let us Hadron Collider at CERN (Nat. Phys. 13, Dark Energy Survey maps these effects not forget Vera Rubin — an obituary 217–223; 2017). to determine the expansion history of by Neta Bahcall was recently published Indirect detection methods involving the Universe, explains Joshua Frieman in Nature (542, 32; 2017). Her galactic astrophysical sources look for imprints (Nat. Astron. 1, 0066; 2017). Meanwhile, rotation curves in the optical range, of dark matter on the energy spectra or Keith Vanderlinde writes about a radio measured with Kent Ford in the 1970s, spatial distribution of gamma-ray photons interferometer, the Canadian Hydrogen helped to convince the community or charged cosmic rays. These particles Intensity Mapping Experiment, which that something was amiss between the come from the Galactic Centre, Milky Way is preparing to scan half the sky, and measured and predicted rotation rates. halo, dwarf spheroidal galaxies, galaxy over 4 billion years of cosmic history, On the modern experimental front, clusters and the extragalactic gamma-ray to probe the increasing expansion rate efforts have focused on sterile neutrinos, background (from pulsars, active galactic (Nat. Astron. 1, 0037; 2017). weakly interacting massive particles nuclei, star-forming galaxies and so forth). The Universe is full of darkness and we (WIMPs) and massive astrophysical Jan Conrad and Olaf Reimer (Nat. Phys. are just beginning to make sense of it. ❒ NATURE ASTRONOMY 1, 0082 (2017) | DOI: 10.1038/s41550-017-0082 | www.nature.com/natureastronomy 1 ©2017 Mac millan Publishers Li mited, part of Spri nger Nature. All ri ghts reserved. .