Unusual observations of galaxies raise questions about our current understanding of dark matter.

Unusual observations of distant galaxies are prompting cosmologists to reconsider established theories about the universe, including the existence of dark matter. A study published on June 20 in *The Astrophysical Journal Letters* suggests that these observations might challenge the current understanding of dark matter.

Richard Brent Tully, an astronomer at the University of Hawaii at Manoa, who was not involved in the study, notes that the findings “raise questions of an extraordinarily fundamental nature.”

The concept of dark matter emerged from observations of the rotation speeds of stars and other visible material at the edges of galaxies. According to existing theories of gravity, these speeds are higher than expected based on the visible mass alone, implying the presence of an unseen mass—dark matter—exerting additional gravitational influence.

The new study explores this by examining gravitational lensing, a phenomenon where massive objects like galaxies warp the fabric of space-time, bending and distorting light from more distant galaxies. To investigate this, astronomer Tobias Mistele from Case Western Reserve University and his team analyzed images from the VLT Survey Telescope at the European Southern Observatory. They searched for signs of gravitational lensing in a catalog of approximately 130,000 galaxies, looking for distortions in light that could indicate the presence of dark matter.

The findings could prompt a re-evaluation of how dark matter is understood or challenge its existence altogether.

The amount of gravitational lensing observed around galaxies is used as a proxy to estimate their masses, including both visible matter and the presumed larger amounts of dark matter surrounding them. The team analyzed this lensing data to determine the mass distribution at various distances from each galaxy’s center and to infer the orbital speeds of stars at those distances.

According to the prevailing cosmological model, Lambda CDM, dark matter is thought to clump into large halos around galaxies. These halos are believed to extend at least 300,000 light-years from a galaxy’s center, and beyond this range, the rotational speeds of stars should decrease.

However, using their gravitational lensing data, Mistele and his team found that stars located up to 3 billion light-years from a galaxy’s center appear to rotate much faster than expected, given both the visible and inferred dark matter. This suggests that either there is more dark matter than previously thought or that alternative theories might be needed.

One such alternative is modified Newtonian dynamics (MOND), which proposes that gravity behaves differently at the galactic scale, eliminating the need for dark matter. This theory, supported by Mistele’s coauthor Stacy McGaugh, aligns with some of the observations made in this study.

Despite these challenges, Lambda CDM remains a robust model. Bhuvnesh Jain, a cosmologist at the University of Pennsylvania, argues that the extensive evidence supporting dark matter makes it premature to discard it entirely. He suggests that more advanced theoretical models, possibly inspired by higher-dimensional theories such as string theory, might reconcile current observations with the role of dark matter.

Future data from the European Space Agency’s Euclid satellite, which launched last year, is expected to provide more detailed gravitational lensing measurements. This could help clarify the ongoing debate and refine our understanding of dark matter and galactic structure.