This is akin to drawing a precise map of what we cannot see: a team of astronomers has just established the finest representation to date of dark matter, this enigmatic component of the cosmos that cannot be observed directly.
This unprecedented image, obtained thanks to the James Webb Space Telescope, covers a portion of the sky representing about two and a half times the size of the full Moon. The researchers counted nearly 800,000 galaxies there, a total well above that of previous studies. Their method relies on measuring the distortions of light from distant stars, an effect caused by the gravitational influence of dark matter, comparable to viewing through a distorting lens.
This detailed map shows the distribution of dark matter in the COSMOS region, with curves indicating areas of equal density.
Credit: Dr. Gavin Leroy/Professor Richard Massey/COSMOS-Webb collaboration This work supports the idea that this invisible substance has been a major actor in the formation of the cosmos. From the most remote epochs, it aggregated to form gravitational wells, then attracting ordinary matter that forms stars and galaxies. Without this contribution, the large structures we see today probably could not have appeared.
The major interest of this map lies in its remarkable definition, offering an unprecedented level of detail. It demonstrates in particular that the dense areas in dark matter coincide with the regions where visible matter is found, a correlation that goes beyond mere chance. This precision was made possible thanks to the capabilities of the Webb telescope's instruments, notably its infrared detector.
Scientists now plan to use other observatories, such as the European Space Agency's Euclid telescope, to extend this mapping to the entire celestial vault. The ambition is to better understand the characteristics of this substance and trace its evolution over billions of years.
For one of the authors, this map highlights the architect function of dark matter, which progressively structured the visible cosmos. It shows how an imperceptible element made possible the birth of galaxies, stars, and, ultimately, planets like ours.
Map of dark matter produced using data from the James Webb Telescope.
Credit: Dr. Gavin Leroy/Professor Richard Massey/COSMOS-Webb collaboration
Thus, this research does not only deliver a clearer image. It also provides a useful reference base for future investigations into the composition and past of our Universe. The studied area will serve as a comparison point for the next mappings, helping scientists refine their models.
The phenomenon of gravitational lensing
To detect dark matter, astronomers exploit an effect predicted by the theory of general relativity. When the light from a distant galaxy passes near a significant concentration of mass, such as a cluster of dark matter, its trajectory is slightly bent. This is what is called gravitational lensing.
This effect allows to betray the presence of invisible matter. By analyzing how the light from background galaxies is altered, researchers can reconstruct the distribution of the mass responsible for this deformation. The more marked the deformation, the stronger the concentration of matter, even if it emits no radiation.
The fineness of the measurements depends on the performance of the instruments. The James Webb Telescope, thanks to its high sensitivity, is capable of discerning very tenuous alterations, allowing a mapping of dark matter with unmatched definition. This technique was applied to several hundred thousand galaxies to produce the new map.