SIR

We were disconcerted to see that the experiment XENON10 was the main focus of your News Feature 'Welcome to the dark side' on searches for particle dark matter (Nature 448, 240–245; 2007), even though it is still in its infancy. The well-established DAMA/NaI experiment, on the other hand, was confined to a box titled 'Contested results'.

Experiments searching for dark-matter particles must be able to discriminate between a dark-matter signal and background signals created by other particles. Because the number of dark-matter particles hitting Earth is expected to vary during the course of a year, one strategy is to look for an annual variation (with many specific features) in the frequency of peculiar events registered by a detector. Running successfully for seven years, the DAMA/NaI experiment, with its 100-kilogram sodium iodide target, has detected such a peculiar annual variation, the properties of which meet those expected of a dark-matter signal with a confidence level of more than six standard deviations.

It has been proved quantitatively that no effect could produce this signal apart from dark-matter particles. Also, the DAMA effect is not contradicted by the results of other experiments because these experiments take a different approach, and so are unable to investigate the same effect.

The DAMA effect has been proved to be compatible with the most popular dark-matter candidate: the neutralino. Moreover, the DAMA experiment is sensitive to physical scenarios and candidates other than the neutralino — to which other experiments are either poorly sensitive or even blind.