Physicists have long known of four fundamental forces of nature: gravity, electromagnetism, the strong nuclear force, and the weak nuclear force.

Now, they might have evidence of a fifth force.

The discovery of a fifth force of nature could help explain the mystery of dark matter, which is proposed to make up around 85 percent of the universe’s mass. It could also pave the way for a unified fifth force theory, one that joins together electromagnetic, strong and weak nuclear forces as “manifestations of one grander, more fundamental force,” as theoretical physicist Jonathan Feng put it in 2016.

The new findings build upon a study published in 2016 that offered the first hint of a fifth force.

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In 2015, a team of physicists at Hungary’s Institute for Nuclear Research was looking for “dark photons,” which are hypothetical particles believed to “carry” dark matter. To catch a glimpse of these strange forces at work, the team used a particle accelerator to shoot particles through a vacuum tube at high speeds. The goal was to observe the way isotopes decay after thrust into high-energy states — anomalies in the way particles behave could suggest the presence of unknown forces.

Read more: Big Think