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The evidence for the presence of a sort of unseen matter—now dubbed dark matter—is overwhelming half a century after Vera Rubin and Kent Ford showed that it is necessary to account for the spinning of galaxies.

Despite the fact that dark matter interacts with ordinary matter only through gravity, there is so much of it out there—85 percent of all matter in the universe—that it has played a key role in shaping everything we can see, from our own Milky Way galaxy to the wispy gas filaments that connect galaxies across vast distances.

Kevork Abazajian, a theoretical physicist and astrophysicist at the University of California, Irvine, says, “We assume it exists because there’s evidence for it on many, many scales.”

Dark matter has been proposed in a variety of forms, ranging from planet-sized objects known as MACHOs to individual particles such as WIMPs (weakly interacting massive particles of the size of a proton) and even tinier entities like axions and sterile neutrinos.

Scientists began classifying hypothetical dark-matter particles as cold, warm, or hot in the 1980s as a method to make sense of the expanding collection. These classifications are based on how rapidly each sort of dark matter would have gone across the early universe—a speed that would have been determined by its mass—and how hot its surrounds were when it first appeared.

Hot dark matter is made up of light, quick particles; cold dark matter is made up of heavy, sluggish particles; and warm dark matter is in the middle.

WIMPs are cold, sterile neutrinos are warm, and relic neutrinos from the early cosmos are hot, according to this perspective. (Axions are a unique situation since they are both light and incredibly cold.) We’ll take care of them later.)


What is the significance of their speed?

“If a dark matter particle is lighter and quicker, it may go further in a given amount of time, smoothing out any existing structure along the way,” Abazajian explains.

Slower, colder types of dark matter, on the other hand, would have aided in the formation of structure, and based on what we know and observe now, they must have been there.

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Constructing galaxies

Although there are ideas regarding when and how each sort of dark-matter contender evolved, scientists only know for certain that dark matter existed roughly 75,000 years after the Big Bang. According to Stanford theoretical physicist Peter Graham, matter began to take precedence over radiation at that point, and little seeds of structure began to develop.

In the same way that high-energy particle collisions at sites like the Large Hadron Collider give rise to strange new forms of particles, most types of dark-matter particles would have been generated by collisions with other particles in the hot, dense soup of the baby cosmos. Dark-matter particles would have been hot, warm, or cold as the cosmos expanded and cooled, and there may have been more than one type.

According to Abazajian, scientists characterize them as “streaming” freely across the cosmos. However, this description is a bit deceptive. “These things are not simply in one area and then in another place,” he explains, comparing them to leaves floating down a river, all heading in the same direction in a coordinated manner. “They’re everywhere and heading in every direction,” says the narrator.

Each sort of dark matter would have had a particular influence on the evolution of structure along the way, either increasing its clumpiness and hence facilitating the formation of galaxies, or hindering it.

The WIMP, for example, would have been a clump-builder if it had been cold dark matter. It traveled slowly enough to clump together and form gravitational wells, trapping neighboring particles of matter.

Hot dark matter, on the other hand, would have been a clump-smoother, racing through those gravitational wells at such a quick rate that it would have been oblivious to them. According to Silvia Pascoli, a theoretical physicist at the University of Bologna in Italy, if all dark matter was heated, none of those seeds could have evolved into larger structures. That’s why scientists now assume that hot dark-matter particles like relic neutrinos from the early days of the universe might only make up a fragment of the total amount of dark matter.

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“I say that relic neutrinos are now the only known component of dark matter,” Pascoli continues, despite their little contribution. They have a significant influence on the universe’s evolution.”

Warm dark matter, you might assume, would be the ideal dark matter, filling the cosmos with just-right structure like a Goldilocks bowl. Sterile neutrinos are the most likely candidates in this group, and they may theoretically make up the vast bulk of dark matter.

But, according to Abazajian, who studied how particular forms of neutrino oscillations in the early cosmos may have created sterile neutrino dark matter as a doctoral student, much of the parameter space—the sets of conditions—where they could exist has been ruled out.

Although the same oscillations may occur now, he claims, the chances of a normal neutrino becoming sterile through standard oscillations in space are regarded to be very low, with estimates ranging from one in 100,000 to one in a billion trillion.

“To count up to 100 trillion hits in your detector without missing the one hit from a sterile neutrino, you’d have to have a very good counting system,” Abazajian explains.

However, there are a few projects out there that are giving it a shot, relying on novel tactics rather than direct blows.


There’s also the axion

Axions, unlike the other dark-matter possibilities, would be both exceedingly light—so light that their associated fields might spread over kilometers—and extremely cold, according to Graham. They are so loosely connected to other types of matter that scarcely any would have resulted from the frenetic collisions of particles in the early universe’s thermal bath.

“They would have been made differently than the other dark matter contenders,” Graham explains. “Axions would have been very cold at birth and would stay frigid forever, even if the cosmos was quite hot at the time, implying that they are completely cold dark matter.”

“They are essentially not moving,” Graham adds, despite the fact that axions are so light. “They live very near to absolute zero, the temperature where all motion ceases.” They’re like a phantom fluid through which everything else flows.”

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Dark matter of various forms is being hunted for

Some scientists believe that more than one sort of dark matter will be required to account for what we see in the cosmos.

The quest for dark matter has extended in recent years, since tests aiming at detecting WIMPs and creating dark matter particles through collisions at the Large Hadron Collider have so far come up empty-handed. Technological advancements and innovative ways that could push far lighter and even more exotic dark-matter particles out of hiding have aided the spread of search concepts.

Some of these initiatives take use of the clumpiness that dark matter helped to create.

While a postdoc at the US Department of Energy’s SLAC National Accelerator Laboratory, Simona Murgia, an experimentalist at the University of California, Irvine, led a team seeking for traces of collisions between WIMPs and their antiparticles with the Fermi Gamma-ray Space Telescope.

She’s now part of an international team of scientists that will use the world’s largest digital camera, which is being built at SLAC, to perform a massive scan of the Southern sky from the Vera C. Rubin Observatory in Chile. One of the goals of this study is to gain a better understanding of the distribution of dark matter in the universe by observing how it bends light from visible galaxies.

“In a completely different approach, it will tell us something about the nature of dark matter,” Murgia explains. “The more clumpy its distribution is, the more compatible it is with notions that dark matter is cold,” says the researcher.

Over the course of ten years, the camera will take photographs of around 20 billion galaxies, from which scientists aim to deduce the nature of the dark matter that created them.

“We don’t only want to know if there’s dark matter,” Murgia explains. “We want to comprehend cosmology, but we also want to know what dark matter is,” says the researcher.

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