Dark matter is the opposite of gravity

Dark matter and dark energy remain a mystery

About 95 percent of the cosmos consists of dark matter and dark energy, about which little is known so far. Two international research teams used experiments to search for the particles and fields that make up these mysterious components of the universe - without success. The experiments thus draw new limits for theoretical explanations of dark matter and dark energy, according to the scientists in the journal "Science".

Atomic interferometer

Observations of the movement of galaxies and galaxy clusters as well as the investigation of the cosmological background radiation show that the normal matter of which stars, planets and also living beings are made up of only 4.9 percent of the total mass and energy of the cosmos. Without the gravity of dark matter, which contributes 26.8 percent, galaxies and galaxy clusters would not stick together. And dark energy, with a share of 68.3 percent, ensures that the expansion of the cosmos is not slowed down by the attraction of matter, but on the contrary even accelerates.

In the standard model of particle physics there is no explanation for the dark cosmic components. The physicists therefore assume that dark matter consists of previously unknown elementary particles beyond the standard model, for example so-called WIMPs, weakly interacting particles. With the help of special detectors it should be possible to detect extremely rare reactions between such particles and the components of normal matter. Such a signal would have to show an annual modulation due to the movement of the earth relative to dark matter.

In fact, the DAMA / LIBRA experiment in the Gran Sasso underground laboratory in Italy has found such a signal in recent years, but it could not be confirmed by other experiments. This contradiction can be overcome by special forms of dark matter, the particles of which could be detected with DAMA / LIBRA, but not with the other detectors. However, the XENON100 experiment, which is also located in the Gran Sasso laboratory, now removes the basis for these explanations: the researchers are not able to find any signal that fits these explanations with much greater accuracy. Both the dark matter itself and the DAMA / LIBRA results thus remain a mystery.

And dark energy, too, continues to elude direct experimental access. So-called chameleon theories, according to which dark energy is a field whose effect depends on the local mass density, are very popular with researchers. A team of researchers from the University of California at Berkeley has now tried to detect such a chameleon field using an atomic interferometer - without success. The researchers were able to improve the theoretical limits for chameleon theory by about a thousand times compared to previous experiments. Further improvements to the experiment could soon make it possible to completely exclude chameleon fields, according to the team. The researchers also point out that a number of other explanations for dark energy should lead to similar effects in an atomic interferometer as chameleon fields. The experiment also results in severe restrictions for such alternative approaches. As for dark matter, physicists still lack an experimentally proven explanation for dark energy.