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The intensity of cosmic background radiation constitutes a rich storehouse of information about the Universe and its contents, both seen and unseen. At near-optical wavelengths, it guides our understanding of the way in which galaxies formed and evolved with time. Integrating over the known galaxy population, with assumptions about evolution based on Hubble Deep Field data, one obtains levels of extragalactic background light that are within an order of magnitude of existing observational upper limits (and tentative detections), depending on cosmological parameters. For any realistic combination of parameters, the intensity of this background light is determined to order of magnitude by the age of the Universe, which limits the amount of light that galaxies have been able to produce. Expansion darkens the night sky further, but only by a factor of two to three, depending on the details of galaxy evolution and the ratio of dark matter to dark energy in the Universe.

The new era of "precision cosmology" has brought us closer to knowing just what this ratio is. The Universe appears to consist of roughly three parts vacuum-like dark energy and one part pressureless cold dark matter, with a sprinkling of hot dark matter (neutrinos) that is almost certainly much less important than cold dark matter. Baryons -- the stuff of which we are made -- turn out to be mere trace elements by comparison, truly a "second Copernican revolution" in cosmology. The observations do not tell us what dark energy or dark matter are made of, nor why these ingredients exist in the ratios they do, a question that is particularly nagging since their densities evolve so differently with time. At present it simply seems that we have stumbled onto the cosmic stage at an extraordinarily special moment.

At wavelengths other than the optical, the spectrum of background radiation contains an equally valuable wealth of information on the dark components of the Universe. The leading candidates are unstable to radiative decay, or interact with photons in other ways that give rise to characteristic signatures in the cosmic background radiation at various wavelengths. Experimental data on the intensity of this background therefore tell us what the dark matter and energy can (or cannot) be. It cannot be dark energy decaying primarily into photons, because this would lead to levels of microwave background radiation in excess of those observed. The dark matter cannot consist of axions or neutrinos with rest energies in the eV-range, because these would produce too much infrared, optical or ultraviolet background light, depending on their lifetimes and coupling parameters. It could consist of supersymmetric weakly interacting massive particles (WIMPs) such as neutralinos, but data on the x-ray and gamma-ray backgrounds imply that these must be very nearly stable. The same data exclude a significant role for primordial black holes, whose Hawking evaporation produces too much light at gamma-ray wavelengths. Higher-dimensional analogs of black holes known as solitons are more difficult to constrain, but an analysis based on the integrated intensity of the background radiation at all wavelengths suggests that they could be dark-matter objects if their masses are not larger than those of galaxies.

While these are the leading candidates, the same methods can be applied to many others as well. We mention some of these here without going into details. Some of the baryonic dark matter could be bound up in an early generation of stars with masses in the 100 - 105 Modot range. These objects, sometimes termed very massive objects or VMOs [457], are primarily constrained by their contributions to the infrared background [458]. Warm dark-matter (WDM) particles in the keV rest-energy range, including certain types of gravitinos and sterile neutrinos, would help to resolve problems in structure formation and would leave a mark in the x-ray background [459, 460]. Decaying dark-matter particles might have partially reionized the Universe at high redshifts, helping to explain the unexpectedly large optical depth inferred from recent CMB observations [461, 462, 463, 464]. In a similar vein, the recent detection of 511 keV gamma-rays from the Galactic bulge by the INTEGRAL satellite [465] has been interpreted as evidence for a population of annihilating [466] or decaying [467] light dark-matter particles with rest energies of 1 - 100 MeV.

Numerous proposals have involved superheavy particles along the lines of "WIMPzillas" [468], very massive, non-thermal WIMPs with rest energies in the 1012 - 1016 GeV range whose decays would be seen at the upper reaches of the diffuse gamma-ray spectrum [469] and might be responsible for otherwise puzzling observations of ultrahigh-energy cosmic rays [470]. Variations on this theme include strongly-interacting WIMPzillas or "SIMPzillas" [471], gluinos [472] and axinos [473] (the supersymmetric counterparts of gluons and axions), leptonic WIMPs or "LIMPs" [474], "superWIMPs" [475] (superweakly interacting WIMPs whose existence would only be betrayed by the decays of their parent particles, the next-to-lightest SUSY particles) and electromagnetically-coupled or "EWIMPs" [476]. All these particles would affect primarily the gamma-ray portion of the EBL spectrum.

High-energy gamma-rays also provide the best hunting-ground for the dark-matter candidates that arise generically in recent theories involving more than four spacetime dimensions [444]. In brane-world models [477], where gravity propagates in a higher-dimensional bulk while all other fields are restricted to the four-dimensional brane, the graviton possesses a tower of massive Kaluza-Klein excitations or Kaluza-Klein gravitons which carry energy out of supernovae cores before eventually decaying into photon pairs and other particles. Radiative decays are particularly conspicuous near ~ 30 MeV, and the observed EBL intensity at this energy currently sets the strongest experimental constraints on brane-world scenarios with two and three extra dimensions [478, 479]. Massive brane fluctuations or "branons" are also dark-matter candidates whose annihilations would show up in the gamma-ray background [480]. In "universal-extra-dimensions" (UED) models, where all fields can propagate in the bulk, the lightest Kaluza-Klein particle or LKP (no longer necessarily related to the graviton) becomes a natural dark-matter candidate [481]; such particles have a long history and were originally known as "pyrgons" [482]. They too turn out to be sharply constrained by their annihilations into gamma-rays [483, 484, 485]. Higher-dimensional string and M-theories imply the existence of other superheavy metastable states (with such names as "cryptons," "hexons," "pentons" and "tetrons") which could also be the dark matter as well as being responsible for ultrahigh-energy cosmic rays [486, 487].

All these possibilities are particularly interesting since it is quite likely that the puzzles surrounding dark matter and dark energy will not be fully understood until they are situated in the context of a fully unified theory of all the interactions, including gravity. In our view such a theory will almost certainly involve extra dimensions. A rich new field of possibilities thus opens up for nature's most versatile dark-matter detector: the light of the night sky.


For comments and discussions on dark matter and dark energy over the years we thank many colleagues including S. Bowyer, T. Fukui, W. Priester, S. Seahra and R. Stabell.

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