DARWIN is a next-generation dark matter and neutrino observatory based on 50 tons of xenon. Its central TPC of 2.6 m diameter and height is operated as dual-phase detector with optimized light and charge read-out. It will allow to search for WIMPs at the GeV-TeV mass scale down to the "neutrino floor" where coherent interactions of astrophysical neutrinos start to dominate the interaction rate. The experiment will also hunt for solar axions, galactic ALPs and sterile neutrinos in the keV mass range. This is complemented by a high-sensitivity search for neutrinoless double beta decay of Xe-136 and a high-precision measurement of the solar pp-neutrino flux.
This requires excellent signal-background discrimination while maintaining an extrordinary low level of background. A key challenge therein is the efficient purification of the LXe target from the radioisotopes Kr-85 and Rn-222 by cryodistillation and other means, as well as identification and suppression of neutron-induced reaction. Finally, an efficient charge read-out requires electron lifetimes in LXe on the ms-scale. The talk describes the physics reach and design of DARWIN, and focuses on ongoing R&D works on background suppression and improvements of the light and charge read-out in the very large TPC.