This `Focus on Dark Matter' cluster of articles is the very first in a series of featured topics to appear in New Journal of Physics that will be of great interest to the physics community. The idea is to bring together contributions from leading researchers in topical fields to provide insights into the key issues for both experts and non-specialists alike.

Investigations of dark matter in the universe are of fundamental interest to astronomers, astrophysicists, cosmologists, and nuclear and particle physicists. Recent developments have provided new insight into the matter budget of the universe. Using supernovae of type 1a as standard candles, distance measurements indicate an accelerating expansion of the universe. This result suggests a non-zero cosmological constant, and the resulting vacuum energy density contributes 70% of the critical mass of the universe. Gravitational lensing, originally proposed by Einstein and later utilized for mass determination of galactic clusters by F Zwicky and others, has recently been used to map dark matter in clusters and large-scale structures. These experiments show that 30% of the critical mass is contained in these structures, of which only 3% is visible. Since the baryonic component of matter is only 6% of the critical mass, assuming a Hubble constant of 60 km s^-1 Mpc^-1, there is five times more dark matter than baryonic matter in the universe. Prime candidates for making up this deficit are neutrinos with mass (hot dark matter) and weakly interacting massive particles, the so-called WIMPs (cold dark matter). Cosmologists and particle physicists favour the latter to be the lightest supersymmetric particle, the so-called neutralino, since if it does exist it would solve two problems at the same time, namely cold dark matter and grand unification theories. Cosmic background microwave radiation data and large-scale structure distributions, however, favour a cocktail of 30% hot and 70% cold dark matter.

Treumann's group investigates matter distributions in galactic clusters, including massive neutrinos and collisional intracluster gas emitting x-ray radiation. The issue of baryonic dark matter in our galactic halo is explored in the contribution from De Paolis and coworkers. They show that the observed diffuse gamma-ray emission from the galactic halo can be explained by cosmic ray proton scattering on clouds of dark clusters of brown dwarfs and cold self-gravitating H₂ clouds in the halo. New experimental results on terrestrial WIMP searches via WIMP-Xe inelastic scattering are presented by Bernabei's group. The contribution from Cebrián's team describes present and future WIMP experiments using conventional and cryogenic detectors in the Canfranc underground laboratory. An interesting novel large-mass, low-background, superheated droplet detector for WIMP searches is described by Collar and colleagues.

Focus on Dark Matter Contents

Neutrino dark matter in clusters of galaxies

R A Treumann, A Kull and H Böhringer

Gamma ray emission from a baryonic dark halo

F De Paolis, G Ingrosso, Ph Jetzer and M Roncadelli

Cold dark matter searches at the Canfranc underground laboratory

S Cebrián, E García, D Gonzalez, I G Irastorza, A Morales, J Morales, A Ortiz, A Peruzzi, J Puimedon, M L Sarsa, S Scopel and J A Villar

Prospects for SIMPLE 2000: a large-mass, low-background superheated droplet detector for WIMP searches

J I Collar, J Puibasset, T A Girard, D Limagne, H S Miley and G Waysand

Improved limits on WIMP-¹²⁹Xe inelastic scattering

R Bernabei, P Belli, R Cerulli, C J Dai, G Ignesti, A Incicchitti, F Montecchia and D Prosperi

Klaus Pretzl
Laboratory for High Energy Physics, University of Bern, Switzerland

Additional Information on Dark Matter
The Focus on Dark Matter articles address one of the most important questions in modern cosmology, astronomy, astrophysics, nuclear and particle physics. These articles bring together contributions from leading researchers, providing an overview for experts as well as for non specialists. There is a wealth of additional information also available. A selection of related information from IOPP books, magazines and other IOPP journals is shown below.

Books ...
Dark Matter in Astrophysics and Particle Physics 1998: Proceedings of the Second International Conference on Dark Matter in Astro and Particle Physics, held in Heidelberg, Germany, 20-25 July 1998, ed H V Klapdor-Kleingrothaus and L Baudis. This book contains the invited papers presented at the second international conference which brought together leading researchers from around the world to review recent progress and future directions for research in the different approaches to the dark matter problem. Read more about this book at bookmark.iop.org. Special Offer - 25% discount (whilst stocks last) - available only to readers of the NJP 'Focus on Dark Matter' articles. To obtain this discounted price please state on your order form that you read about the book in NJP.

News items from Physics World ...
Boomerang backs flat universe: The faint microwave glow left over from the big bang has been measured with unprecedented precision, giving astronomers a new insight into the nature of the universe...

The search for dark matter: Experiments housed deep underground are searching for new particles that could simultaneously solve one of the biggest mysteries in astrophysics and reveal what lies beyond the Standard Model of particle physics...

Physics gets dark and exotic: Two of the outstanding challenges in physics identified in our millennium survey last month were the nature of "dark matter" and a proper understanding of nuclear structure. This month we look at these challenges in greater detail...

Dark-matter dispute intensifies: Recent results from a dark-matter experiment in Italy suggest that the elusive weakly interacting massive particle or WIMP has finally been detected ­ but a rival experimental collaboration in the US disagrees...

Calling all cosmophysicists: [20 Apr 2000] Over 100 astrophysicists, particle physicists and space scientists attended a workshop on fundamental physics in space organized by the European Space Agency (ESA) and CERN in Geneva last month. The aim of the meeting was to encourage greater interactions between the different communities with a view to generating proposals to test the fundamental laws of physics in space...

Dark matter claim meets resistance: [25 Feb 2000] Physicists working at the Gran Sasso underground laboratory claim to have observed the first direct evidence for dark matter particles in experiments. The so-called weakly interacting massive particle or WIMP has a mass about 50 times the mass of the proton. If confirmed the findings will have immense implications for particle physics and cosmology...

Articles from other IOPP journals ...
Non-baryonic dark matter: observational evidence and detection methods.
Lars Bergström
2000 Rep. Prog. Phys. 63 793-841
The evidence for the existence of dark matter in the universe is reviewed. A general picture emerges, where both baryonic and non-baryonic dark matter is needed to explain current observations...