  Observations of galaxies and primordial radiation suggest that the Universe
is made mostly of non-luminous dark matter. Several types of new fundamental
particles have been proposed as candidates for dark matter such as weakly
interacting massive particles (WIMPs) but no definitive signal has been seen
despite concerted efforts by many collaborations. One exception is the
much-debated claim by the DAMA collaboration of a statistically significant
annual modulation in the event rate of their experiment with a period and phase
consistent with that expected from WIMP dark matter. Several groups have been
working to develop experiments with the aim of reproducing DAMA's results using
the same target medium. Here we report results from the initial operation of
the COSINE-100 experiment. COSINE-100 uses sodium iodide as the target
medium-the same medium as DAMA-and is designed to carry out a model-independent
test of DAMA's claim. Initial data based on the first 59.5 days indicate that
there is no excess of events over the expected background, confirming that
DAMA's annual modulation signal is in severe tension with results from other
experiments under the assumption of dark matter having spin independent
interactions and the Standard Halo Model. COSINE-100 is now taking data to
study the presence of dark matter-induced annual modulation in the event rate
of the sodium iodide detectors.
