What is observed by the Hubble Space Telescope and the VLT, in Chile, do not coincide with computer models and reveal that “an ingredient” is missing in the theories to explain dark matter

The latest observations from the Hubble Space Telescope and the Very Large Telescope (VLT) in Chile have revealed that an “ingredient” is missing from current theories on dark matter, and scientists have not been able to find it until now.

This ingredient could also explain the reason for an unexpected discrepancy between observations of the distribution of dark matter in massive galaxy clusters and theoretical simulations of how dark matter should be distributed within these clusters. The new findings indicate that some small-scale concentrations of dark matter produce gravitational lensing effects that are up to ten times stronger than expected. The results of this research have just been published in Science.

Although we cannot see it, since it does not emit radiation and therefore is not detectable by our instruments, it is known that dark matter is the “glue” that holds stars, dust and gas together in a galaxy. In fact, this mysterious substance accounts for most of the mass of galaxies and constitutes the basis on which the Universe in which we live is structured. However, and because dark matter does not emit, absorb or reflect radiation, its presence is only known by the gravitational influence it exerts on matter that we can see in space. Physicists and astronomers have spent decades trying to determine exactly what this mysterious type of matter is, five times more abundant than the “normal” matter that shapes the planets, stars and galaxies we can see.

Dark matter deposits

Large galactic clusters, clusters of galaxies that are the most massive structures in the Universe, are also the largest deposits of dark matter. Clusters, in fact, are made up of individual galaxies that are largely held together by the gravity of the dark matter.

“Clusters of galaxies are ideal laboratories to study if computer simulations of the Universe faithfully reproduce what we can infer from gravitational lenses”, explains Massimo Meneghetti, lead author of the study.

A gravitational lens is formed when light from a distant object is bent as it passes near very massive objects, such as a galactic cluster. In doing so, and as if it were a huge space magnifying glass, the objects that scientists observe are distorted and magnified, appearing closer. This phenomenon allows astronomers to study remote galaxies that would otherwise be too weak to see.

“We have carried out a lot of tests with the data from this study,” Meneghett continues, “and we are sure that this mismatch indicates that some physical ingredient is missing in the simulations or in our understanding of the true nature of dark matter.”

“There is a characteristic of the real Universe that we are simply not looking at in our current theoretical models,” says Priyamvada Natarajan, from Yale University and one of the theorists in the study. “This could indicate a gap in our current understanding of the nature of dark matter and its properties, as these exquisite data has allowed us to probe the distribution of dark matter on the smallest scales in detail.”

The glue of the Universe

The distribution of dark matter in galactic clusters is obtained by measuring how light curves due to the gravitational lensing effect that these clusters produce. The gravity of the dark matter concentrated in the clusters increases and deforms, in fact, the light that comes from objects more distant from the cosmic background. Which results in distortions in the shapes of the background galaxies that appear in the images of astronomers. Gravitational lenses can often also produce multiple images of the same distant galaxy.

Obviously, the more matter there is in a cluster, the more dramatic this light-deflecting effect will be. And the presence of smaller agglomerates of dark matter inside galaxy clusters further increases the level of distortions. In a way, as has been said, the galaxy cluster itself acts like a large-scale magnifying glass that has many other smaller magnifying glasses embedded inside it.

Using the images from the cameras of the Hubble and VLT telescopes, the team of researchers managed to produce an accurate and detailed map of dark matter in the clusters studied. By measuring the distortions caused by gravitational lenses, in effect, astronomers were able to determine the amount and distribution of dark matter in the three main clusters: MACS J1206.2-0847, MACS J0416.1-2403 and Abell S1063.

In this way, and to the surprise of scientists, in addition to the expected deformations in each cluster produced by gravitational lenses, an unexpected amount of distorted images of smaller-scale also appeared near the nucleus of each cluster, right where the most massive individual galaxies are located. The researchers believe that these small “nested” lenses are the result of the gravity of dense concentrations of matter within the galactic clusters themselves. Measuring the speed of the stars that orbit within several of these galaxies made it possible to determine their masses.

“Hubble and VLT data,” says Piero Rosati, another team member, “providing excellent synergy. We were able to associate galaxies with each cluster and estimate their distances.”

Dozens of deformed galaxies

By combining the images from the two telescopes, astronomers were thus able to identify dozens of background objects whose images were distorted or multiplied by the lenses. This allowed them to assemble a high-resolution map of the distribution of dark matter within each cluster.

After collecting this observational data, researchers compared them with simulations of the dark matter distribution in galaxy clusters with masses and distances similar to those observed. And the computer models did not show the same level of dark matter concentration.

So something is missing from the theories. An element today unknown but which, in the light of this work, has not been taken into account and could alter much of what we know, or think we know, about how dark matter behaves and distributes in the Universe. Researchers say they will continue to investigate and deepen the results of this work. They may finally reveal the true nature of this mysterious and essential kind of matter.