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The quest for dark matter is entering an exciting phase. Astrophysicists and cosmologists keep on telling us particle physicists that a large amount of dark matter is certainly required, and that it is probably mainly cold. There are still some nagging worries about this paradigm, related to galactic cores and the absence of observable small satellite galaxies, but these seem resolvable [100] and there is no serious rival to the cold dark matter paradigm [94]. However, astrophysicists cannot tell us what this dark matter is composed of: that is the task of we particle physicists.

We certainly have plenty of candidates, ranging from axions through supersymmetric particles to cryptons. Which if any of these is correct can only be decided by particle experiments, either with accelerators or using astrophysical sources. The greatest accelerator hopes lie with the LHC, but non-accelerator experiments have an almost free rein until it starts taking data in 2007, and some candidates such as axions and cryptons lie beyond the reach of accelerators.

Dark energy is even more of a challenge than dark matter. Particle physicists did not expect it, and do not have many convincing ideas for its origin. A deeper understanding of quantum gravity is surely necessary.

Given this state of ferment, there is plenty to keep us all busy until the next Dark 200N meeting!

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