There was a lot of talking in the science community about the dark matter. However, it is only hypothetical kind of matter – it cannot be observed as it is invisible, it does not emit nor absorb light or any other electromagnetic radiation at any significant level, and it was never directly observed in the laboratory.
But there is international science collaboration, called XENON, which includes researchers from the Universities of Bern and Zurich, searching for evidences of the existence of dark matter. And although they still have not been successful in proving that dark matter actually exists, recently they solved a long-standing scientific controversy in the field.
Of course, science is not about guessing. Dark matter was hypothesized because of some qualities of the universe that allot presuming there is something else we cannot observe. In fact, dark matter constitutes a significant part of the matter in our universe – it accounts for approximately 63% of all the matter in our universe. However, all evidences about it are indirect and scientists can only presume it exists because of its gravitational pull on luminous matter.
There were claims about discovery of the particles of dark matter. 14 years ago, scientists from DAMA/LIBRA collaboration announced discovery of the enigmatic dark matter particles, but science community has always been sceptical about these findings. Now another international collaboration group XENON may have proved that these results were not accurate and dark matter still is not proved.
Scientists used extremely sensitive XENON100 detector, but still have not discovered any dark matter particles yet. Such detectors work on assumption that dark matter particles would reveal themselves by occasionally colliding with atomic nuclei in the detector. Because of rotation of Earth around the Sun, chances to catch the signal are seasonal – scientists are expecting to catch dark matter more in summer than winter.
In the DAMA/LIBRA experiment scientists have measured precisely such a seasonal modulation with their Sodium-Iodine detector. However, other experiments did not prove that such modulation is because of dark matter. Theorists have postulated that dark matter particles would not scatter off atomic nuclei as the central assumption of many experiments suggests. Instead, they would interact with the electrons of the atomic shell.
DAMA/LIBRA experiments cannot distinguish between interactions with atomic nuclei and electrons. Therefore, XENON collaboration analysed its data for signatures of dark matter interactions with electrons, which requires the best possible equipment.
The XENON100 detector is one of the most sensitive devices known to science. It uses 62 kg of liquid xenon as detection medium and measures the tiny charge and light signals, which are expected to be induced by the rare collisions of dark matter particles with xenon atoms. Since XENON detector is more advanced, it can distinguish well between interactions with nuclei and electrons, but the location to place this device is also very important. It is being carried out in the Italian Gran Sasso Underground Laboratory, where the cosmic ray flux is reduced by 1400 m of rock.
Scientists have looked for scatters off electrons and for temporal variations, but, in contrast with DAMA/LIBRA experiments, XENON team have not detected any significant modulation for periods up to 500 days. In order to be sure how detector would react to particles of dark matter, scientists did calculations and now they are sure that all they observed was some background noise. Therefore, now they say that DAMA/LIBRA did not detect any dark matter. However, it does not mean that existence of dark matter has been denied completely.
Marc Schumann, one of the authors of these studies, explains that it does not mean that dark matter does not exist. He said – “You have to imagine the dark matter particles as small fish in the ocean. We know that they are there, but our nets are simply not yet sufficiently fine-meshed to catch them.”
And it looks like scientists are ready to make this mesh much finer. Currently they are installing a 100 times more sensitive detector than XENON100, which reached its limit. Scientists say that this new detector, called XENON1T, will start a new era for dark matter searches by the end of this year. So we can only wait for new results and hope that this new “fishing net” will be fine enough to catch dark matter.