The universe is packed with riddles, but few are as stubborn—or as fascinating—as dark matter. First proposed in 1933 by astronomer Fritz Zwicky, this elusive substance refuses to play by the rules: it doesn’t shine, absorb, or interact with light in any way. In fact, we can’t see it at all. And yet, its invisible pull shapes galaxies, hinting that something massive—and mysterious—is out there.
After nearly 100 years, and with help from NASA’s Fermi Gamma-ray Space Telescope, researchers may have finally “seen” dark matter for the first time.
If this proves to be true, it’ll be a significant development for science. Dark matter’s ability to hide in plain sight is legendary. It can’t be seen by any tool humans have ever made because dark matter can’t emit, absorb or reflect light of any kind, which is how humans and all of our tools see things. That makes dark matter impressively difficult to find.
Tomonori Totani, an astronomy professor at the University of Tokyo, believes he may have succeeded where so many before him have 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 particles of dark matter colliding with one another.
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The key to this discovery is the theoretical existence of something called weakly interacting massive particles, or WIMPs for short. WIMPs are pieces of dark matter that are larger than protons and don’t interact with any other types of particles. When two WIMPs collide with each other, scientific theory suggests that they annihilate one another, and the resulting reaction produces gamma rays.
Totani used data from NASA’s Fermi Gamma-ray Space Telescope to find what he believes are the 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.
Scientists theorize that roughly 27% of the universe’s total mass-energy is made up of dark matter.
Why is dark matter so difficult 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 that you and I can see, whereas dark matter makes up 27% of the pie. The rest is dark energy, which is yet another mystery that science has yet to solve.
If there’s more than five times as much dark matter as there is regular matter, then why is it so hard to see? The short answer is that dark matter doesn’t interact with matter in a way that humans can detect with our current technology.
This isn’t entirely unnatural. Science also has a tough time detecting black holes. Light cannot escape a black hole, so it is impossible to observe one directly. Instead, scientists have developed several methods to detect the presence of a black hole based on its impact on the surrounding environment.
Cygnus X-1 — the first black hole ever detected — was found thanks to something called an accretion disk. Accretion disks are swirling clouds of gas, dust, plasma, and other particles that form around black holes and tend to emit vast amounts of X-ray radiation. Researchers found 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.
English philosopher and clergyman John Michell first theorized the existence of black holes in 1783. That means it took humankind 236 years to take a picture of a black hole, and even then, we can’t see the black hole in the picture. We just know it’s there because we can see its accretion disk.
Dark matter is much more challenging to detect. It doesn’t interact with the electromagnetic spectrum at all, including visible light. Much like black holes, science has used its impact on its environment to try and prove its existence.
This phenomenon began in 1933, when astronomer Fritz Zwicky observed that galaxies in the Coma Cluster were moving too quickly for the amount of ordinary matter contained within it. Zwicky concluded that there must be a second type of unseen matter that was adding more gravitational force, acting as a sort of glue that held the cluster together.
This theory has been refined over time, with additional evidence emerging. One example is gravitational lensing, which is a bending of light caused by gravity. The Bullet Cluster is the best example of this being potentially caused by dark matter, but it has not yet been definitively proven.
The gravitational lensing around the Bullet Cluster (shown here in blue) is one of the clearest potential examples of dark matter’s gravitational effects on light.
Study author 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 characteristic. When they collide, science predicts that they will destroy one another, resulting in a burst of gamma rays.
NASA has a short video here that shows 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 electronvolts, a huge amount of energy, extending in a halolike structure toward the center of the Milky Way galaxy,” Totani told Phys.org. “The gamma-ray emission component closely matches the shape expected from the dark matter halo.”
There’s a little to unpack here, so I asked Totani for more information. He told me that stars in our galaxy are “distributed in a disk, while the dark matter halo is thought to surround it spherically.” The radiation generated from the theoretical dark matter would reach into 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 dark matter predictions.”
To put it another way, 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 shape expected for dark matter.
NASA posits that the dark ring around the CL0024+17 cluster may be dark matter.
Changing science forever
Totani found gamma rays where they were supposed to be and at the strength predicted, 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 to have independent researchers verify Totani’s conclusions.
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 emissions from other sources, such as dwarf galaxies, in the universe to see if something else can explain his findings.
Currently, his findings can’t be easily explained by any known sources of gamma ray emissions, but that doesn’t mean that none exist. The data will need to be tested and retested, and researchers will need to bring in more information to verify that his findings are indeed related to dark matter.
Science will take its time with this, because if Totani actually did find dark matter, the ramifications would be massive. He notes that the discovery of a new elementary particle not included in the current Standard Model of particle physics will have a significant impact on fundamental physics theory. And the discovery of dark matter would help piece together other cosmological mysteries, such as the nature of dark energy, the invisible force that causes the universe to expand at an accelerated rate.
“If correct, the true nature of dark matter, long the greatest mystery in cosmology, has been revealed,” Totani said.
