The hunt for dark matter—and the associated particles and forces that we expect to accompany it—has turned up numerous false dawns over the years. Try as we might, any evidence of what makes up this invisible form of matter—thought to be the vast majority of matter in the known universe—has remained elusive. But a team of Hungarian researchers suggested in 2015 that they had found a particle, dubbed X17, that possibly interacted with dark matter in some way. Recently, in a second experiment, the team says it has fresh evidence for the X17 particle, which would change physics as we know it. But not everyone is convinced, and new experimental plans are afoot to root out the truth.

In a paper posted on the preprint server arXiv.org, which has not yet been peer-reviewed, Attila Krasznahorkay of the Institute for Nuclear Research (also known as Atomki) at the Hungarian Academy of Sciences and his colleagues report the new findings. Back in 2015 the team observed the decay of beryllium-8 nuclei and found that pairs of electrons and positrons (electrons’ antimatter counterparts) ejected in the process were consistent with the additional decay of a mysterious extra particle, the X17 particle, with a mass of about 17 million electron volts (MeV). Now the researchers say they have seen evidence for this particle again but this time in the decay of helium-4 nuclei. “We studied the decay of high-energy nuclear states, first of all in beryllium-8 and then, more recently, in helium-4,” Krasznahorkay says. “We got a short difference between the prediction and the experimental data, an anomaly. In order to understand this anomaly, we assumed a new particle, which is created in the atomic nucleus and ejected, then decays to electron-positron pairs.”

The experimental setup at Atomki involves bombarding a target with protons to examine its nuclear decay. This technique is somewhat different than the usual methods of particle detection such as at the Large Hadron Collider (LHC) at CERN near Geneva, which smashes particles together at high energies and observes the resultant particles emitted in the collision. The Atomki process, however, provides a unique method to look for unexpected particles. The scale of 17 MeV is “difficult to probe using the Large Hadron Collider, which tends to be operating at a much higher energy,” says Jesse Thaler of the Massachusetts Institute of Technology Center for Theoretical Physics, who was not involved in the experiment.

Since the Hungarian team published its first paper back in 2015, other scientists have tried and failed to find evidence for the X17 particle. An outside analysis in 2016, however, suggested that if this particle truly exists, it could be evidence for a supposed “fifth force” of nature, specifically related to dark matter. “This fifth force really means there is a new particle that intermediates new interactions, or new forces,” says Daniele Alves, a particle physicist at Los Alamos National Laboratory, who was not involved in the Hungarian team’s work. “It’s possible that this particle is part of a larger ‘dark sector,’ meaning it could also interact with dark matter particles. It could be a portal to this sector.”

Matt Strassler, a theoretical physicist at Harvard University, who also wasn’t involved in Hungarian team’s studies, notes this particle would be “a window into some aspect of the universe that we are completely unaware of,” with huge implications. “Not only would it be obviously Nobel Prize–winning because it would be a new [fundamental] particle, but this particular particle doesn’t fit into the existing table of particles,” he says. “The Standard Model, all of the existing elementary particles, form sort of a closed book. [But] X17 interacts with matter much more weakly. And that’s an indication that it’s not part of the structure of the Standard Model. Its interactions with matter are through some other story we don’t know yet.”

The existence of this particle is anything but a certainty, however. The fact that only the Hungarian team has been able to spot it so far has raised alarm bells for some scientists, suggesting the data could be explained by a fault in its experimental setup. And Strassler notes that the supposed properties of the particle require it to have some odd characteristics. “To set up mathematical equations where you have a particle that interacts with neutrons and electrons more than with protons and neutrinos turns out to be not so simple to do,” he says. “This just makes the story somewhat implausible.”

That assessment does not mean other scientists are not trying to look for X17, however. The NA64 collaboration at CERN has previously tried and failed to find signs of the particle. But when upgrades to the LHC are complete in the next year or so, scientists plan to use the Large Hadron Collider beauty (LHCb) experiment, which studies another particle known as the beauty quark, to see if the mysterious X17 turns up. “The LHCb experiment, based on our study, should have been able to collect enough data [by 2023] in order to make a definitive statement about the X17 [particle],” Thaler says.

Alves and her colleagues are exploring the possibility of using Los Alamos to search for the particle, too. “We are investigating whether some of the studies that Los Alamos does for other purposes could also be repurposed to look for signs of this new particle,” says Alves, who notes that their method of searching would be somewhat different from the Hungarian team’s. “The Hungarians looked at two nuclear transitions, one in beryllium-8 and one in helium-4,” she says. “We will be looking at the production of particles in neutron-capture reactions, when a neutron is captured by another nucleus, and in the process, it might emit things. The most common thing it might emit is a photon, but it could also possibly emit this new X17 particle.”

Although skepticism remains high, there is still considerable excitement at the possibility of the X17 particle. It may take years until we know for sure if it really exists, but if it does, it would herald an entirely new branch of physics and a chance to peer into the unknown. “Of course, I’m confident [that it exists],” Krasznahorkay says. “But I’ve got strong critics.”