dark matter--2/11/2026

11 Feb 2026

dark matter--2/11/2026

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Today's selection-- from Space Oddities by Harry Cliff. Dark matter, dark energy, and black holes:

“As scientists grapple with long-standing puzzles like the Hubble tension and the peculiar magnetism of the muon, new and strange phenomena are constantly being uncovered. This embarrassment of scientific riches is what makes the pursuit of anomalies so endlessly fascinating, helping us refine both how we do science and our understand-ing of nature. So, before we draw our story to a close, here are a couple more emerging enigmas that may prove significant in the coming years.

“Alexandra Amon is a young cosmologist at Cambridge's Kavli Institute for Cosmology, a group of pavilions set among leafy parkland on the edge of the city. She is at the center of an anomaly story that, like the Hubble tension, is threatening the standard cosmological model.

Amon grew upon the tiny Caribbean islands of Trinidad and Tobago before moving to the U.K. to study at the University of Edinburgh. There she worked on the Canada-France-Hawaii Telescope Lensing Survey (CFHTLenS), an international project that used a huge telescope perched on the summit of Mauna Kea, a dormant volcano in Hawaii, to map dark matter. You might very well wonder how a telescope can be used to map dark matter, given that, famously, it's completely invisible. Dark matter does, however, leave its mark on the visible universe, in particular through an effect known as gravitational lensing.

“According to Einstein's general theory of relativity, gravity is not a force but a manifestation of the curvature of space and time. Massive objects like stars and galaxies bend the fabric of space-time around them, like a bowling ball plonked on the surface of a trampoline. As a result, when light passes close to massive objects, its path, too, must be bent as it travels through the curved space. This means that anything with enough mass, say a big cloud of dark matter, can act like a lens, distorting how visible galaxies and stars appear in the night sky.

“Surveying around ten million galaxies, the CFHTLenS team found that many looked as though they had been slightly squashed in one direction or another as their light was lensed by dark matter on its way to Earth. By mapping where these distortions were greatest, the team was able to reveal vast web-like structures of dark matter threading their way through the cosmos at the grandest scales imaginable. This enormous cosmic web is the invisible skeleton on which our visible universe hangs. More than that, its gravity was responsible for drawing matter together after the big bang to form the first galaxies and stars.

“However, the final results of the CFHTLenS project sug-gested something unexpected: these vast structures were less clumpy, for want of a better word, than the standard cosmological model predicted. As with the Hubble tension, measurements of the cosmic microwave background allow cosmologists to determine the properties of the early universe, in particular how evenly or unevenly matter was distributed in the fireball following the big bang. Then, using the standard cosmological model, they can run the clock forward to predict how these primordial seeds should have grown as their gravity pulled in more and more matter to form the giant structures we see in the universe today.

Dark matter map for a patch of sky based on gravitational lensing analysis of a Kilo-Degree Survey

“The problem is, these predictions suggest that the universe ought to be denser and more clumpy than it appears. Or to put it another way, like a watery soup, the universe is simply too thin.

“Initially, cosmologists were skeptical of the team's result, but during Amon's PhD more and more observations began to confirm their findings. As Amon told me when we met, ‘I grew up hearing that CFHTLenS was wrong. Then the story kind of changed and people were like, 'Huh, lensing surveys are all low, not just CFHTLenS: So then it changed from just CFHTLenS being wrong to all lensing being wrong.’

“Amon now leads the lensing team working on the Dark Energy Survey, a major international project that spent six years mapping more than 300 million galaxies in an attempt to better understand how dark energy shapes the universe. When they unblinded their results, at the end of 2020, they found that they too were in tension with the predictions of the standard cosmological model. As with many of the results we've seen, their tension was only at around the two- to three-sigma level, still far below a formal discovery. But taken together with other results, they seemed to be strongly pointing to something strange going on in the cosmos.

“In 2021, Amon moved to Cambridge to work with George Efstathiou, one of the world's most eminent cosmologists and an architect of the standard cosmological model. Efstathiou is an arch skeptic when it comes to the Hubble tension and has engaged in a long running debate with Adam Riess, arguing that it's far more likely to be the product of missed experimental effects than the harbinger of the fall of the cosmological model. When Amon arrived in Cambridge, he had a similar attitude toward the new tension over the dumpiness of the universe. That was at least part of why she wanted to work with him.

“However, a year and a half on, she told me that he's been won over. So much so that they recently put out a paper together in an attempt to explain what could be making the universe more spread out than we’d expect. There were two broad explanations.

“The first is to do with the most powerful objects in the universe: supermassive black holes. These behemoths are found at the centers of galaxies and can weigh tens of billions times more than our Sun. Their enormous gravitational pull draws gas and dust from the surrounding area into glowing disks of superheated matter that shine with the light of countless stars. As the black hole devours material from the disk, it can produce indescribably powerful jets of energy that blast outward across thousands of light-years and heat any surrounding gas to high temperatures, which has the knock-on effect of preventing stars from forming in any region unlucky enough to end up in their firing line. Supermassive black holes therefore block structure formation in the universe, making it less clumpy overall. Astrophysicists have done their best to estimate how big such an effect may be, but there is a chance that it's much larger than thought.

“That would be the ‘boring’ explanation for the anomaly, but frankly it would still be pretty exciting to learn more about these incredible objects. The interesting explanation, on the other hand, relates to the nature of dark matter.

“The standard cosmological model assumes that dark mat-ter is made up of a single type of massive particle. However, this rather simple picture need not be true. It could well be that there are multiple types of particles in the dark universe, just as our visible world is made up of several fundamental ingredients. This is what Amon and Efstathiou's work seems to be hinting at. She explained, ‘I think it's on the dark matter side; I think there's going to be some paradigm shift. When I first heard about multicomponent dark matter, I just thought, “That's even more weird; why would that be a thing?” But I've changed my mind now. Even George [Efstathiou] thinks it's more likely to be something to do with dark matter.’”