Scientists may have finally seen dark matter

The universe has no shortage of mysteries, many of which have puzzled us for centuries. One of the biggest is the existence of something called dark matter. First theorized in 1933 by Fritz Zwicky, dark matter is a theoretical type of matter that cannot be seen because it does not interact with light or any other form of electromagnetic radiation.

After nearly 100 years, and with the help of NASA’s Fermi Gamma-ray Space Telescope, researchers may have finally “seen” dark matter for the first time.

If this turns out to be true, it will be a significant development for science. Dark matter’s ability to hide in plain sight is legendary. It cannot be seen by any instrument that humans have ever made because dark matter cannot emit, absorb or reflect light of any kind as humans and all of our instruments can see. That makes dark matter incredibly hard to find.

Tomonori Totani, a professor of astronomy at the University of Tokyo, believes he may have succeeded where so many before him had failed. In a study published Nov. 25 in the Journal of Cosmology and Astroparticle Physics, Totani says he may have found dark matter by observing the byproduct of two dark matter particles colliding with each other.

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Key to this discovery is the theoretical existence of something called weakly interacting massive particles, or WIMPs for short. WIMPs are bits of dark matter that are larger than protons and do not interact with any other type of particle. When two WIMPs collide with each other, scientific theory suggests that they annihilate each other, and the resulting reaction produces gamma rays.

Totani used data from NASA’s Fermi Gamma-ray Space Telescope to find what he believes are gamma-ray emissions from these annihilation events, which, if accurate, would prove that dark matter exists — or at least put scientists on the right track to confirming its existence.

Why is dark matter so hard to find?

NASA describes dark matter as “the invisible glue that holds the universe together.” Dark matter is everywhere. Theories suggest that only 5% of matter is the ordinary stuff you and I can see, while dark matter makes up 27% of the pie. The rest is dark energy, which is still a mystery that science has yet to solve.

If there is five times as much dark matter as ordinary matter, then why is it so hard to see? The short answer is that dark matter does not interact with matter in a way that humans can detect with our current technology.

This is not completely unnatural. Science also has a difficult time detecting black holes. Light cannot escape from a black hole, so it is impossible to observe it directly. Instead, scientists have developed several methods to detect the presence of a black hole based on its impact on its surroundings.

Theories suggest that only 5% of matter is the ordinary stuff you and I can see, while dark matter makes up 27% of the pie.

Cygnus X-1 — the first black hole ever detected — was found thanks to something called an accretion disk. Accretion discs are swirling clouds of gas, dust, plasma and other particles that form around black holes and tend to emit large amounts of X-rays. Researchers detected those intense X-rays and concluded that they came from a black hole. In the first photo of a black hole taken in 2019, the visible part is the black hole’s accretion disk, not the black hole itself.

The English philosopher and clergyman John Michell first theorized the existence of black holes in 1783. That means it took humanity 236 years to photograph a black hole, and even then we can’t see the black hole in the image. We only know it’s there because we can see its accretion disk.

Dark matter is much more difficult to detect. It does not interact at all with the electromagnetic spectrum, including visible light. Like black holes, science has used its impact on its environment to try to prove its existence.

This phenomenon began in 1933, when the astronomer Fritz Zwicky noticed that galaxies in the Coma cluster they were moving too fast for the amount of ordinary matter contained in it. Zwicky concluded that there must be a second type of unseen matter that adds more gravitational force, acting as a kind of glue that held the cluster together.

This theory has been refined over time as additional evidence has emerged. An example is gravitational lensing, which is a bending of light caused by gravity. The Bullet Cluster is the best example where this can be caused by dark matter, but it has yet to be definitively proven.

The author of the study explains what he found

Over the decades, scientists have proposed various potential candidates for what dark matter particles actually are. One such theory is the WIMP. These theoretical particles are much larger than photons and have a distinctive feature. When they collide, science predicts they will destroy each other, resulting in a gamma-ray burst.

NASA has a short video here showing how this would work in theory. These gamma-ray emissions are what Totani believes he has found.

“We detected gamma rays with a photon energy of 20 gigaelectronvolts (or 20 billion electron volts, a huge amount of energy) extending in a halo-like structure toward the center of the Milky Way galaxy,” Totani told Phys.org. “The gamma-ray emission component closely matches the shape expected from a dark matter halo.”

There’s more to unpack here, so we asked Totani for more information. He told me that the stars in our galaxy are “distributed in a disk, while the dark matter halo is thought to surround it spherically.” The radiation generated by the theoretical dark matter would reach the disk from its spherical location, giving Totani an idea of ​​what to look for and where to look in general.

Once he looked there, he was able to find radiation that he says is “consistent with predictions about dark matter.”

In other words, the gamma rays were where they were supposed to be, at the photon energy level that science predicted they would be, and the emissions were in the form expected for dark matter.

Changing science forever

Totani found the gamma rays where they were supposed to be and at the predicted strength, so it must be dark matter, right?

Not exactly.

While these findings are promising, they do not necessarily prove the existence of dark matter. The first step will be for independent researchers to verify Totani’s findings.

Totani is aware of this and wants independent researchers to examine the data in an attempt to replicate his findings. This includes measuring gamma-ray emission from other sources, such as dwarf galaxies, in the universe to see if something else can explain its findings.

“If correct, the true nature of dark matter, long the greatest mystery in cosmology, has been revealed.” Tomonori Totani, professor of astronomy at the University of Tokyo

At present, his findings cannot easily be explained by any known source of gamma-ray emission, but that does not mean that they do not exist. The data will need to be tested and retested, and researchers will need to come up with more information to verify that his findings are indeed related to dark matter.

Science will take its time with this, because if Totani really found dark matter, the ramifications would be massive. He notes that the discovery of a new elementary particle that is not included in the current standard model of particle physics will have a significant impact on the fundamental theory of physics. And the discovery of dark matter would help unify other cosmological mysteries such as the nature of dark energythe invisible force that causes the universe to expand at an accelerated rate.

“If correct, the true nature of dark matter, long the biggest mystery in cosmology, has been revealed,” Totani said.