Dark matter in the universe: Where, what, and why?

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Abstract
The universe is pervaded by non-luminous matter. Observations at many wavelengths, and on many length scales, yield a reasonably good picture of the amount of dark matter and its distribution. In very broad terms, the larger the scale we survey, the larger the fraction of gravitating mass that does not emit its fair share of light. The range is from about 50% in the solar neighbourhood (the nearest few hundred parsecs?) to 99% or more in the largest clusters and superclusters of galaxies (ten million or more parsecs across). Observations do not, so far, tell us what that dark matter is made of, or even whether it is all the same kind of thing. Candidates that cannot currently be ruled out include tiny stars, stellar remnants, some kinds of black holes, neutrinos with rest masses 10−5 to 10−4 of the electron mass, and still more exotic kinds of particles (photinos, gravitinos, axions, majorons, Higgsinos—) that interact at most weakly with normal matter.