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What is the Universe made of?

 Only 4% of the Universe is made of ordinary matter. Following the latest measurements and cosmological models, 73% of the cosmic energy budget seems to consist of "dark energy" and 23% of dark matter. The nature of dark energy remains a mystery, probably intimately connected with the fundamental question of  the "cosmological constant problem".

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Dark matter turns out to be the majority component of cosmic matter. It holds the Universe together through the gravitational force but neither emits nor absorbs light. Dark matter (including a small admixture of massive neutrinos) has likely played a central role in the formation of large scale structures in the Universe. Its exact nature has yet to be determined. The discovery of new types of particles which may comprise the dark matter would confirm a key element of the Universe as we understand it today. The favoured candidate for particulate dark matter is the lightest supersymmetric (SUSY) particle, most probably the neutralino. Astroparticle physicists have developed a variety of tools for direct and indirect neutralino searches and will explore a large fraction of the best motivated theoretical models. These explorations will complement  SUSY searches at the Large Hadron Collider, LHC.  An alternative possibility is that dark matter consists of axions, light pseudoscalar particles copiously produced in the Early Universe, or of bosonic particles with axion-like interactions. Other particles beyond the standard model of particle physics may contribute on a smaller level to the cosmic inventory, such as magnetic monopoles or extremely heavy SUSY states. Last but not least, the extent of matter-antimatter asymmetry is explored by searches for antiparticles and tested against theories of the early Universe.
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