Ever since astronomers realized that most of the matter in the universe is invisible, they have tried to sort out what that obscure stuff might be. But three decades of increasingly sophisticated searches have found no sign of dark matter, causing scientists to question some of their basic ideas about this elusive substance.

In October the most sensitive experiment looking for proof of the leading candidate for dark matter—theorized particles called WIMPs (weakly interacting massive particles)—reported null results, disappointing scientists once again. Now some researchers are reexamining dark matter candidates once written off as unlikely, and considering less satisfactory ideas such as the possibility that dark matter will turn out to be made of something more or less undetectable.

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