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- Galaxies and galaxy clusters move faster than they should.<\/strong> That\u2019s why scientists proposed dark matter: an invisible substance whose mass would add an extra pull of gravity. But \u2026 we\u2019ve never directly detected dark matter.<\/li>\n
- So what if the laws of gravity as we know them don\u2019t apply across space?<\/strong> One theory \u2013 called MOND, or Modified Newtonian Dynamics \u2013 suggests gravity behaves differently over vast distances.<\/li>\n
- Now, by measuring gravity across hundreds of millions of light-years<\/strong>, a new study suggests it does behave the same on cosmic scales as close to home. It appears to behave as Isaac Newton and Albert Einstein found.<\/li>\n
- The new study strengthens the case for unseen dark matter<\/strong> existing in large amounts across space.<\/li>\n<\/ul>\n
This story came originally from the University of Pennsylvania. Edits by EarthSky.<\/p>\n
Science news, night sky events and beautiful photos, all in one place.<\/strong> Click here to subscribe to our free daily newsletter.<\/p>\n
Gravity works across the universe<\/h3>\n
Gravity, as Isaac Newton described it, is the familiar force that pulls a falling apple toward Earth. <\/p>\n
Or, as described by Albert Einstein, gravity is a property of the fabric of spacetime. The greater the gravity, the more spacetime curves.<\/p>\n
For decades, cosmologists have wondered if the laws of gravity \u2013 as described by Newton and Einstein \u2013 apply everywhere across space. Puzzling observations of unusually fast-moving galaxies have forced cosmologists like Patricio A. Gallardo of the University of Pennsylvania to revisit the fundamentals of physics. For example, these scientists explore whether the laws of gravity as described by Isaac Newton in the 1600s and Albert Einstein in the early 1900s truly apply everywhere. Gallardo said: <\/p>\n
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Astrophysics has been plagued by a massive discrepancy in the cosmic ledger. When we look at how stars orbit within galaxies or how galaxies move within galaxy clusters, some appear to be traveling way too fast for the amount of visible matter they contain.<\/p>\n<\/blockquote>\n
This mismatch forces a choice between two radical conclusions. So according to Gallardo:<\/p>\n
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Either the universe contains concentrations of massive invisible \u2018dark matter\u2019 that provide extra gravitational pull or the fundamental equations for gravity need to be modified.<\/p>\n<\/blockquote>\n
So Gallardo and his team set out to answer the question: do the laws of gravity as described by Newton and Einstein hold true across cosmic distances? Their recent study suggests the answer is yes. <\/p>\n
Testing gravity across galaxy clusters<\/h3>\n
Researchers made observations with the Atacama Cosmology Telescope (ACT), a telescope developed largely by Penn researchers led by Mark Devlin. Gallardo and collaborators tested gravity across galaxy clusters separated by hundreds of millions of light-years. <\/p>\n
Their findings, published in the peer-reviewed journal Physical Review Letters<\/em> on April 15, 2026, show that gravity\u2019s strength weakens with distance. And those results are almost exactly as predicted by the equations developed by Newton and later incorporated into Einstein\u2019s theory of general relativity. Gallardo said:<\/p>\n
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It is remarkable that the law of the inverse of the squares \u2013 proposed by Newton in the 17th century and then incorporated by Einstein\u2019s theory of general relativity \u2013 is still holding its ground in the 21st century.<\/p>\n<\/blockquote>\n
The confirmation that gravity behaves as predicted by the established theory over vast, extragalactic distances reinforces a fundamental pillar of modern science. Gallardo explains:<\/p>\n
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By showing that fundamental theories of gravity do not break down on the largest scales, the data effectively closes the door on a group of theories such as Modified Newtonian Dynamics (MOND), that attempt to explain cosmic motions by modifying the laws of gravity.<\/p>\n
When Newton proposed the inverse square relation, which states that gravity weakens in proportion to the square of the distance between objects, he was primarily concerned with describing the movements of objects in the solar system. This same principle has now been tested on masses and distances that were inconceivable in Newton\u2019s day.<\/p>\n<\/blockquote>\n
Understanding the universe\u2019s speed limits<\/em><\/h3>\n
The universe\u2019s galaxies \u2013 of which there are more than 200 billion \u2013 don\u2019t move the way gravity alone says they should.<\/p>\n
Following Newtonian logic, stars farther from a galaxy\u2019s center should orbit more slowly. Instead, astronomers see the opposite. The outermost regions move faster than visible matter can account for. The same mismatch appears in galaxy clusters, where entire galaxies move too quickly for their mass.<\/p>\n
![\"Above:](\"https:\/\/earthsky.org\/upl\/2026\/04\/cosmic-background-passing-through-galaxy-clusters-Lucy-ReadingSimons-Foundation.png\")
The study used the kinematic Sunyaev-Zel\u2019dovich, or kSZ, effect, a tiny change imprinted on the cosmic microwave background when its light passes through hot gas around moving galaxy clusters. Researchers used the data to measure how quickly pairs of clusters are drawing together, and to test whether gravity weakens with distance the way standard physics predicts. Image via Lucy Reading\/ Simons Foundation\/ University of Pennsylvania.<\/figcaption><\/figure>\n Gallardo explained:<\/p>\n
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That is the central puzzle. Either gravity behaves differently on very large scales, or the universe contains additional matter that we cannot directly see.<\/p>\n<\/blockquote>\n
Testing gravity across the cosmos<\/h3>\n
To test this, the researchers turned to ACT\u2019s observations of light released about 380,000 years after the Big Bang. It\u2019s been traveling across the universe ever since. It\u2019s known as the cosmic microwave background.<\/p>\n
As this ancient light passes through massive galaxy clusters, it is subtly altered by their motion. And it leaves behind faint imprints that astronomers can detect. By reading these distortions and measuring these motions across hundreds of thousands of clusters separated by tens of millions of light-years, the researchers determined how strongly gravity pulls on the largest structures in the cosmos. If modified gravity theories such as MOND were correct, the measurements would reveal a flatter gravitational fall-off.<\/p>\n
Instead, the results landed almost exactly where both Newton\u2019s and Einstein\u2019s theories agree.<\/p>\n
Because that prediction holds, the missing mass problem cannot be explained by changing gravity itself. That strengthens the case that an unseen component \u2013 dark matter \u2013 must be providing the extra pull.<\/p>\n
The dark matter mystery<\/h3>\n
Understanding what dark matter is remains one of the biggest challenges in modern physics. According to Gallardo:<\/p>\n
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This study strengthens the evidence that the universe contains a component of dark matter. But we still do not know what that component is made of.<\/p>\n<\/blockquote>\n
Future observations of the cosmic microwave background and larger galaxy surveys will allow physicists and astronomers to test gravity even more precisely.<\/p>\n
Gallardo concluded:<\/p>\n
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With so many unanswered questions, gravity remains one of the most fascinating areas of research. It\u2019s a naturally attractive field.<\/p>\n<\/blockquote>\n
Bottom line: Newton and Einstein\u2019s laws of gravity hold true across the largest structures in the universe. This new evidence strengthens the case for dark matter.<\/p>\n
Source: Test of the Gravitational Force Law on Cosmological Scales Using the Kinematic Sunyaev-Zeldovich Effect<\/p>\n
Via University of Pennsylvania<\/p>\n
<\/div>\n
\n - So what if the laws of gravity as we know them don\u2019t apply across space?<\/strong> One theory \u2013 called MOND, or Modified Newtonian Dynamics \u2013 suggests gravity behaves differently over vast distances.<\/li>\n