The vast majority of matter in the universe might be invisible. This dark matter is found everywhere and helps keep galaxies and clusters of galaxies in shape due to its gravitational influence. Its existence is crucial in many theories, and yet we still don’t know what dark matter is.
Many experiments are currently looking for dark matter, but they are yet to observe what it is made of. One experiment, originally called DAMA and now DAMA/LIBRA, has been consistently finding a signal for over two decades – but it’s got some issues. The properties of the possible dark matter seen in this experiment have already been excluded by different detectors, and similar detectors are yet to find the signal.
What DAMA has been observing since 1997 may be seasonal changes in the flux of dark matter. The researchers envision dark matter a bit like a headwind. As the Sun goes around the Milky Way, it moves through dark matter and, depending on the direction our planet is going around the Sun, the researchers expect to see more or less dark matter hitting the detector – and this is what has been seen. More dark matter signals appear in June than in December.
Two new detectors, ANAIS and COSINE-100, designed to work in the same way as DAMA/LIBRA have gone online in the last couple of years. Unfortunately, their preliminary results don’t see the same signals as DAMA. The results of COSINE-100's first three months were published in Nature last December, and the results from the first 18 months of observation from ANAIS are currently available on arXiv.
"For the first time in 20 years, we have a chance to resolve the DAMA conundrum," co-author Yale physics professor Reina Maruyama, who is co-spokesperson for COSINE-100, said in a statement.
While the results create a tension between the experiments, it is still too early to know if they are a true contradiction or a fluke of the numbers. The DAMA results are confirmed by large number statistics, but these new studies will definitely add to the debate around DAMA and its findings. Both experiments expect statistically significant results to be possible in a few years.
The experiments use sodium iodide crystals. When dark matter or regular background particles interact with them, they emit light that is detected by photomultipliers. To get rid of as much background as possible, all three experiments are located deep below ground. DAMA is inside the Gran Sasso mountain in Italy, COSINE-100 is in the Yangyang Underground Laboratory in South Korea, and ANAIS is located at the Canfranc Underground Laboratory in Spain.