For a quarter of a century, dark energy has loomed large as one of the great mysteries of the cosmos. First proposed in the late 1990s, dark energy is meant to explain an unsettling discovery: the expansion of the universe wasn’t slowing down as one might expect — it was speeding up. Galaxies weren’t just drifting apart. They were racing away from each other, driven by a force that no one understood.

Now, a team of astronomers peering deep into the universe’s past believe that force may be weakening.

The finding comes from the Dark Energy Spectroscopic Instrument, or DESI, perched atop a telescope in Arizona’s Kitt Peak National Observatory. Using 5,000 tiny robotic eyes, DESI has scanned the skies with unprecedented precision, mapping 15 million galaxies across 11 billion years. With that massive dataset, astronomers have reconstructed a three-dimensional map of the universe more detailed than ever before.

And hidden in that map is a surprising clue: dark energy may have peaked in strength billions of years ago. Since then, it has apparently weakened by about 10 percent.

“What we’re finding is that, yes, there is something pushing galaxies away from each other, but it is not constant. It is declining,” Prof. Carlos Frenk, a cosmologist at the University of Durham and a member of the DESI collaboration, told The Guardian.

Dark Energy Is Getting Even Stranger

Since the beginning, scientists have assumed dark energy was a constant feature of the universe — a form of Einstein’s infamous “cosmological constant,” a fudge factor he invented in 1917 and later discarded. If true, the universe was headed for a lonely, desolate end known as the “big freeze,” where all matter drifts so far apart that not even light can traverse the void.

But DESI’s results suggest that’s not the path we’re on.

“It’s exciting to think that we may be on the cusp of a major discovery about dark energy and the fundamental nature of our universe,” said Prof. Alexie Leauthaud-Harnett of the University of California, Santa Cruz, a spokesperson for DESI.

If dark energy continues to weaken, the accelerating expansion of the universe could slow — and possibly reverse. In that case, galaxies would begin to fall back together. The universe would collapse in on itself in a fiery end known as the “big crunch.” Physicists sure love an epic finale.

“It’s fair to say this result, taken at face value, appears to be the biggest hint we have about the nature of dark energy in the ~25 years since we discovered it,” Adam Riess, a Nobel laureate in astrophysics who co-discovered dark energy but who was not involved in the new research, told the New York Times.

Conflicting Clues

The results, presented last week at the American Physical Society meeting in California, have not yet passed peer review (but are due to appear in the journal Physical Review D). They fall just short of physics’ gold standard in confidence in a result: five sigma, or a 1-in-3.5-million chance the results are a fluke. Instead, the data sit at 4.2 sigma — a 1-in-50,000 chance.

Still, some experts are already convinced.

“I’m not on the fence,” said Frenk. “To me, this is a robust result. We’re witnessing the overthrow of the old paradigm and the emergence of a new paradigm.”

Others remain cautious.

“My take-home from this analysis is that the … measurements do not yet provide decisive evidence for evolving dark energy,” Prof. George Efstathiou of the University of Cambridge told The Guardian. “They may do as DESI accumulates more data.”

The cosmic waters are further muddied by new results from a completely different telescope — the Atacama Cosmology Telescope (ACT) in Chile. ACT recently released the sharpest maps yet of the universe’s oldest light, the cosmic microwave background (CMB), from when the universe was just 380,000 years old.

Those maps confirm the predictions of the standard model of cosmology, including the idea of a constant dark energy. In other words, the early universe looks exactly as it should.

But ACT’s findings only reflect the distant past, when the Universe was just a ‘baby’ — not the more recent billions of years, where DESI is spotting the apparent change.

The Strange Fate of Everything

To understand the stakes, it helps to recall how dark energy earned its name. In 1998, two teams of astronomers measured the brightness of distant supernovas — stellar explosions so bright they can be seen across the cosmos. These explosions act like cosmic yardsticks. To the teams’ surprise, the supernovas appeared fainter than expected, meaning they were farther away than they should have been, even in an expanding universe. It told the astronomers that the universe wasn’t just expanding — it was accelerating.

That was shocking. The known forces of physics couldn’t explain it. So scientists dubbed the driver “dark energy,” something that now we know makes up about 70 percent of the entire universe — but remains invisible and unexplained.

The implications of the new DESI surveys are staggering. If dark energy is fading, the universe may not be doomed to eternal cold and silence. Instead, it could someday return to a stable state — or collapse in a cosmic implosion.

But no one knows why dark energy would behave this way.

“It’s fair to say we have no idea what dark matter or dark energy is,” said Prof. Ofer Lahav of University College London. “The constant dark energy [theory] is already sufficiently challenging. I feel like: ‘As if things were not complicated enough.’ But you can also look at it more positively. For 20 years we’ve been stuck with dark energy. Now physicists have new questions.”

In the next few years, more data will arrive. DESI continues its survey. Other instruments — from NASA’s upcoming Roman Space Telescope to Europe’s Euclid — will join the hunt. Each will add to the picture, revealing different facets of this elusive force.

For now, dark energy remains as mysterious as ever. But for the first time in decades, there’s a glimmer that something may be changing.

As Prof. Will Percival of the University of Waterloo put it: “This is actually a little bit of a shot in the arm for the field. Now we’ve got something to go after.”