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Summary
In the past 17 years, a new picture of our cosmos has emerged based mainly on the energy-matter budget of the universe, as determined by the percent level by observation (1). A striking consequence is that regular matter built from the known particles of the standard model (e.g., quarks, leptons, neutrinos, and photons) accounts for only 4.9% of the total energy-matter density. Most of our universe consists of two components of unknown origin that are colloquially called dark matter (26.8%) and dark energy (68.3%). Two reports in this issue describe experimental efforts in the search for the proposed constituents. On page 849, Hamilton et al. (2) search for chameleon fields, one of the most prominent dark energy candidates, and on page 851, the XENON Collaboration (3) reports on their search for dark matter.
