A recent study released by the University of Pennsylvania in the United States shows that researchers used the Atacama Cosmological Telescope to conduct the most extensive gravity test to date on galaxy clusters hundreds of millions of light-years apart. The results found that on the ultra-large scale of the universe, the way gravity weakens with distance is still consistent with the inverse square law proposed by Newton and later absorbed and developed by Einstein's general theory of relativity. This result not only provides new support for the standard cosmological model, but also further strengthens the evidence chain for "the real existence of dark matter."
The research team pointed out that gravitational phenomena in daily life may seem intuitive, such as apples falling to the ground and planets orbiting the sun, but on a cosmic scale, gravity is always one of the most critical objects for testing basic physical theories. It determines how galaxies form, how galaxy clusters move, and also shapes the large-scale structure of the entire universe. However, astronomers have long faced a problem in their observations: Many stars and galaxies are moving significantly too fast based solely on the mass of visible matter, and it seems that they cannot be explained by the gravitational pull generated by existing visible matter.
Cosmologist Patricio A. Gallardo, one of the authors of the paper, said this "huge gap in the cosmic ledger" has troubled the astrophysics community for many years. Whether it is the rotation of stars within galaxies or the movement of galaxies in galaxy clusters, it is shown that some celestial bodies move much faster than visible matter allows. Faced with this contradiction, the scientific community usually proposes two explanation paths: first, there is a large amount of invisible dark matter in the universe, which provides additional gravity to these celestial bodies; second, the existing gravity theory needs to be modified on a very large scale.
To test which explanation is closer to the truth, the researchers called on data from the Atacama Cosmological Telescope. Developed by researchers at the University of Pennsylvania, the telescope is an observation equipment about three to four stories high. It is mainly used to measure the cosmic microwave background radiation, which is the faint afterglow left over from the Big Bang. The research team focused on analyzing the subtle changes caused by this ancient light passing through huge galaxy clusters, thereby inferring the motion state of galaxy clusters when they are close to each other, and further testing the actual strength of gravity on ultra-large scales.
The cosmic microwave background radiation was born about 380,000 years after the Big Bang and fills the entire universe. As these light rays pass through regions of galaxy clusters containing hot gas, they experience extremely weak but measurable distortions due to the motion of the cluster. It is by analyzing this signal that researchers conducted statistical studies on hundreds of thousands of galaxy cluster samples spanning tens of millions or even hundreds of millions of light-years to determine whether gravity still decays with distance as predicted by classical theory.
The results show that the observational data are highly consistent with the predictions of Newton's theory and Einstein's general theory of relativity. If alternative theories like Modified Newtonian Dynamics (MOND) are correct, then the pattern of gravity's decay at very large scales should deviate from traditional theoretical expectations; however, this measurement did not detect such a deviation. The researchers therefore believe that, at least on the cosmic scales currently tested, there is no evidence that modifications to the laws of gravity are necessary to explain the observations.
Gallardo said that it is surprising that the inverse square law proposed by Newton in the 17th century can still be tenable on such a grand scale of the universe after entering the 21st century. When Newton discussed this law, he mainly focused on the motion of celestial bodies within the solar system. Today, scientists have expanded and tested this principle to distance scales and mass scales that were unimaginable in his time.
Researchers pointed out that this means that the "missing mass" problem is more difficult to explain with the "failure of gravity theory", but further supports the existence of a material component in the universe that has not been directly observed, that is, dark matter. In other words, if gravity itself does not behave abnormally on large scales, then the additional gravitational effects in galaxies and galaxy clusters are more likely to come from invisible sources of mass rather than from the rewriting of physical laws.
However, the research also emphasizes that the true nature of dark matter remains one of the most important unsolved problems in modern physics. This work strengthens the evidence for "dark matter as one of the components of the universe", but it does not answer what it is actually made of. In the future, as the accuracy of observations of the cosmic microwave background radiation is further improved and larger-scale galaxy survey projects continue to advance, scientists are expected to conduct more precise tests on the laws of gravity and dark matter issues.
This research paper is titled "Testing the Law of Gravitation at the Cosmological Scale Using the Kinematic Sunyaev-Zeldovich Effect" and was published in "Physical Review Letters" on April 15, 2026. The research was completed in collaboration with scholars such as P. A. Gallardo, and the project also received support from the National Science Foundation and other institutions.