Dark matter has had a decisive influence on the development and structure of our universe. But astronomers have so far only been able to determine relatively roughly how this invisible form of matter, which only interacts via gravity, is distributed in the cosmos. Now, with the help of the James Webb Space Telescope, the most precise mapping of dark matter to date has been achieved. The telescope’s high resolution made it possible to evaluate tiny distortions in the light of distant galaxies, such as those caused by the presence of dark matter. The result is a dark matter map that includes twice as much detail as the Hubble Telescope’s most precise mapping to date. It reveals not only large accumulations of dark matter in galaxy clusters and other large-scale structures, but also fine filaments and small halos of dark matter.
Dark matter is almost omnipresent in the universe, making up around 85 percent of all matter. But many of its features remain a mystery, including what particles it is made of. The only thing that seems clear is that this invisible, barely interacting form of matter has shaped the development of the cosmos. According to popular theory, the gravitational pull of dark matter after the Big Bang ensured that the raw material of the first stars, galaxies and large structures collected in certain places. As a result, the gravitational influence of dark matter formed the basic framework for all large-scale structures in the universe – from the huge galaxy clusters and filaments of the cosmic network to the shape and movement of the smallest galaxies. Despite this enormous importance, the exact distribution of dark matter is currently only partially known. Because where there is dark matter and how much can only be determined indirectly.
255 hours of observation time and almost 800,000 galaxies
The common method for mapping dark matter is weak gravitational lensing. Astronomers use the fact that the mass of invisible dark matter bends space-time. When light from distant galaxies, for example, passes through these zones, this curvature creates subtle distortions. By measuring these distortions in the light of distant galaxies, astronomers can draw conclusions about the mass and distribution of dark matter in the space in between. “However, previous mappings based on weak gravitational lensing have been limited by their resolution or sensitivity, leaving the finer dark matter structures underlying the cosmic network invisible,” explain Diana Scognamiglio of NASA’s Jet Propulsion Laboratory in California and her colleagues. To remedy this, they used the high resolution power of the James Webb Telescope to map dark matter in a section of the sky more precisely than ever before.
To carry out their mapping, the astronomers aimed the space telescope’s near-infrared camera (NIRCam) at the so-called COSMOS field, a section of the constellation Sextant about 2.5 full moons in size, for 255 hours. “Located at the celestial equator, this field is visible from both hemispheres. Decades of observations from nearly all major telescopes on Earth and in space have given us a comprehensive view of this field in all wavelengths,” the team explains. This allows a good comparison between the distribution of normal matter and dark matter. The new images from the Webb telescope captured around 800,000 galaxies in the COCMOS field, including many that had not been seen before. Data from the mid-infrared MIRI instrument on the Webb telescope helped determine the respective distances. Using the light from these galaxies, the astronomers were then able to determine the subtle effects of the dark matter lying between the telescope and the galaxies.
Framework of the universe made visible
The result is a map that shows the distribution and mass of dark matter with unprecedented resolution. Because the James Webb Telescope images around ten times more distant galaxies in the COSMOS field than earth-based telescopes and a good two times more than the Hubble Space Telescope, it can show the distribution of dark matter between the telescope and galaxies with a resolution of around one arc minute. “This is the largest dark matter map ever created with the Webb Telescope and is twice as sharp as any previous dark matter mapping by other observatories,” says Scognamiglio. “Before, we only saw a blurry image of dark matter. Now, thanks to the incredible resolution of the Webb Telescope, we see the invisible framework of the universe in astonishing detail.” In addition to large accumulations of dark matter, for example in galaxy clusters, the new dark matter map also shows the network of dark matter bridges that connect these accumulations with each other. “Our technique allows us to simultaneously identify small structures such as the halos of low-mass or distant groups of galaxies or the filaments that connect them,” the astronomers write.
Scognamiglio and her colleagues compared the new dark matter map with the distribution of normal matter and X-ray sources in this section of the sky. This confirmed the close spatial connection between dark and normal matter. “Our map shows how this invisible component of the universe structured visible matter – enabling the formation of galaxies, stars and ultimately life,” says co-author Gavin Leroy from Durham University in England. “The map thus reveals the hidden but essential role of dark matter, the true architect of the universe.” The new dark matter map still only covers a small section of the sky. However, the mapping using the European Euclid Telescope and NASA’s future Nancy Grace Roman space telescope should show much more.
Source: Diana Scognamiglio (Jet Propulsion Laboratory, Pasadena) et al., Nature Astronomy, doi: 10.1038/s41550-025-02763-9