Weak Interacting Massive Particles (WIMPs) are prime candidates for the dark matter in the universe. The ArDM experiment consists of a 1t liquid argon detector to search for WIMPs interacting elastically with argon nuclei. Both scintillation light and ionization charge from nuclear
recoils are measured. The 128 nm light is reflected and shifted into the visible range by wavelength shifting foils and detected by 8" photomultipliers immersed in the liquid. The charge will be detected above the liquid surface by a double stage large electron multiplier. The detector is
being developed at CERN on the surface before being moved to an underground site. ArDM was filled for the first time in spring 2009 and operated for more than one month. Preliminary results lead to a light collection efficiency of about 0.3 photoelectrons / kev for nuclear recoils, which
shows that our goal of reaching a detection threshold of 30 keV can be achieved.