We wish to clarify a few points you make in relation to the dark-matter (DAMA) experiment at Italy's Gran Sasso National Laboratory (Nature 485, 435–438; 2012).

It is not an assumption of ours that Earth's velocity through dark matter varies as it orbits the Sun to produce an annual variation in the flux of dark-matter particles: this property was described many years ago (see, for example, K. A. Drukier et al. Phys. Rev. D 33, 3495–3508; 1986). Our experiments are based on this principle.

The XENON100 experiment, which you quote as failing to detect weakly interactive massive particles (WIMPs) in DAMA's mass range, has not been a “source of tension” for us, for both experimental and theoretical reasons. To list a few, XENON100 uses a different methodology, a different target material and several data-subtraction procedures.

The XENON100 comparisons depend on modelling, whereas our results are model-independent. Also, the sensitivities of the experiments to the many possible dark-matter candidates and to astrophysical, nuclear and particle-physics scenarios are quite different from those of DAMA, including to WIMPs scenarios.

Neither are the XENON100 results backed up by those from CRESST (G. Angloher et al. preprint at http://arxiv.org/abs/1109.0702; 2011) at Gran Sasso, although they too search for recoil-like events in the data.