Dark matter is behaving unexpectedly in way that suggests there is a ‘missing ingredient’, scientists say

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Dark matter is not behaving as we would expected it to, scientists have said.

There is an unusual mismatch between our theoretical models of how dark matter should be spread across galaxy clusters, and the way it actually seems to be behaving in those clusters, the researchers said.

Dark matter remains almost entirely elusive. It cannot be seen directly, but its presence can only be inferred through the way it interacts with other matter that can be seen, and attempting to see its gravitational effects.

One of the ways that researchers have done that is to watch for how that gravity distorts space, which can be seen in an effect called gravitational lensing. The effect of the gravity distorts light as it travels through space, and that light can be seen and used to measure the otherwise invisible dark matter.

As they did so, researchers have found that small-scale concentrations of dark matter in clusters are producing 10 times stronger effects than would normally be expected. That suggests there is something wrong with our theoretical understanding of how that dark matter should behave.

The new findings come from observations of a range of different galaxy clusters, done by Nasa's Hubble Space Telescope and the European Southern Observatory's Very Large Telescope. Those observations are more detailed than ever before.

Watching such galaxy clusters is helpful because they are the largest repositories of dark matter. The giant clusters of different individual galaxies are held together by dark matter's gravity, and each of those individual galaxies have dark matter of their own too, allowing researchers to see it both at the large and smaller scale.

"Galaxy clusters are ideal laboratories to understand if computer simulations of the universe reliably reproduce what we can infer about dark matter and its interplay with luminous matter," said Massimo Meneghetti of the INAF (National Institute for Astrophysics)-Observatory of Astrophysics and Space Science of Bologna in Italy, the study's lead author.

"We have done a lot of careful testing in comparing the simulations and data in this study, and our finding of the mismatch persists. One possible origin for this discrepancy is that we may be missing some key physics in the simulations."

Priyamvada Natarajan of Yale University in New Haven, Connecticut, one of the senior theorists on the team, said in a statement: "There's a feature of the real universe that we are simply not capturing in our current theoretical models. This could signal a gap in our current understanding of the nature of dark matter and its properties, as these exquisite data have permitted us to probe the detailed distribution of dark matter on the smallest scales."

The research is published this month in the journal Science.