Physicists say that 96% of the Universe is unseen, and appeal to the ideas of 'dark matter' and 'dark energy' to make up the difference. In the first of two articles, Jenny Hogan reports that attempts to identify the mysterious dark matter are on the verge of success. In thesecond, Geoff Brumfiel asks why dark energy, hailed as a breakthrough when discovered a decade ago, is proving more frustrating than ever to the scientists who study it.

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