How did dark matter form the universe? This physicist has ideas

At the age of 12, Tracy Slatyer felt sorry for a book. She read a newspaper article about how many people were buying A brief history of time by Stephen Hawking. “But then … nobody was reading it,” she says. “People were just leaving it on their coffee tables.”

Determined to right this wrong, Slayer picked up a copy and diligently read every page. The famous physicist’s popular text revealed to her “that mathematics was in a sense an expressive language for describing how things really work,” she says. “That, for me, was exciting.”

These days, Slatyer, a theoretical physicist at MIT, uses her math skills to dream up new ideas about dark matter. The mysterious substance makes up about 85 percent of the matter in the universe. However, it has repeatedly eluded scientists’ attempts to find it. Slatyer tries to understand where dark matter might come from, how it might interact with itself or with anything else, and, most importantly, the consequences of those interactions.

Physicists know that dark matter exists because they can see its gravitational influence on galaxies, galaxy clusters, and the overall evolution of the universe. Beyond that, there is little data to work with. Slatyer has helped imagine the myriad ways that dark matter could leave subtle signatures on the fabric of reality that would show up in observations.

Among scientists doing such work, “I don’t think there’s been one that’s had more impact,” says Dan Hooper, a physicist at the University of Chicago. “She’s as big a deal as I can make her be.”

Discovery of Fermi bubbles

Born in the Solomon Islands, Slatyer grew up in Canberra, Australia. After her encounter with Hawking’s book, she knew she wanted to study physics. While in graduate school at Harvard University in the 2000s, she met physicist Douglas Finkbeiner, who was investigating mysterious signals at the center of the Milky Way.

A research satellite had noticed strange excesses of positrons, electron antiparticles and high-energy photons called gamma rays that could not be explained by conventional theories. Together, Slatyer and Finkbeiner began looking deeper into a type of self-destructing dark matter that might address the mystery. In their particular model, this dark matter would leave behind electrons and positrons, which would interact with starlight to create gamma rays.

In 2008, NASA launched the Fermi Gamma-ray Space Telescope, which provided unprecedented views of high-energy photons emanating from the galactic jet. If dark matter was truly self-annihilating, it would show up in Fermi’s observations. The following year, Slatyer and Finkbeiner used Fermi’s public data to find out.

“We analyzed the data and saw this big fuzzy glow north and south of the galactic center,” Slatyer recalls. “So we’re like, ‘Victory!’

But the more they and another Finkbeiner student, Meng Su, looked at the signals, the more they realized it wasn’t dark matter. Fermi’s images revealed a large hourglass figure extending 25,000 light-years above and below the plane of the Milky Way. Dark matter is thought to be present in a diffuse halo around our galaxy, but this structure had very sharp edges.

Supermassive black holes that feed on gas and dust at the centers of other galaxies have been known to eject material in hourglass shapes. Finally, Slatyer and her colleagues realized that this might be something similar. These Fermi bubbles, as they became known, have been the subject of numerous subsequent studies, leading to a long-running debate over the mechanisms driving bubble creation (SN: 11/9/10; SN: 4/20/23).

Slatyer hadn’t found dark matter, but, she says, “I try not to complain when nature gives me exciting new things, whether or not they were what I was looking for in the first place.”

Dark matter in the early universe

Much of her work since then has focused on various dark matter scenarios. For example, some of her research has looked at how the mysterious substance might have annihilated or decayed in the early universe, leaving behind fundamental particles that would cause small changes in the expected temperature of the overall cosmos. Such an effect can be seen in the cosmic microwave background, or CMB, a remnant light left over from when the universe was only 380,000 years old.

Satellites measuring this light have found that it shows the cosmos was almost exactly the same temperature no matter which direction they look, with deviations of just one part in 100,000. Slatyer and her colleagues calculated that, if dark matter annihilation had occurred, it could have generated an even subtler temperature signature, down to one part per million. Her team reported in 2023 how the presence of self-annihilating dark matter would distort the CMB—a signal for future instruments to look for.

In a study published in May 2024, she and colleagues looked at other possible effects of excess heat in the early universe from dark matter. In some scenarios, this higher temperature may have generated excess free electrons. Those free electrons could have acted as catalysts for chemical reactions that would have favored star formation, possibly leading to the creation of large numbers of stars very early.

Other teams have suggested that the excess heat would have pushed gas and dust around more easily, a move that may have reduced star formation. In that case, larger clumps of material may have collapsed into massive black holes, which could have become the seeds around which the first galaxies coalesced.

Such ideas may help explain what the James Webb Space Telescope has seen as it looks into cosmic history. The telescope appears to have found black holes and unexpectedly large galaxies in the early universe (SN: 3/4/24). Slatyer and her colleagues are suggesting that dark matter may be the culprit behind these surprisingly massive cosmic objects.

By taking her theories to their logical conclusions, Slatyer has made herself invaluable to the community of theoretical and observational physicists searching for dark matter. “She’s one of these people who is kind of ubiquitous,” Finkbeiner says. “She shows up at every meeting. She has her finger in every pie. She is on every panel to understand what the field needs to do for the next 10 years.”

Given how little researchers know about dark matter, Slatyer thinks it’s important to imagine a wide range of possible possibilities and then come up with experiments to test those options. “We try to … make sure we don’t miss anything glaringly obvious,” she says.