Euclid's First Data Haul: 900 Million Galaxies in 2.5 Trillion Pixels

Here's the breakdown. Everything we can see — stars, planets, galaxies — accounts for only about 5% of the universe's total contents. Dark matter makes up roughly 27%; we can't see it directly, but its gravity helps galaxies form and holds cosmic structures together. Dark energy accounts for about 68% and appears to drive the universe's accelerating expansion.

"These are some of the greatest mysteries in modern physics. And Euclid — alias our dark universe detective — is designed especially with this goal in mind, to help us unravel them," Silvestri says.

What does the telescope actually measure?

Euclid attacks the problem from two directions:

• Galaxy positions. It maps where billions of galaxies sit, revealing how cosmic structures have evolved over time.
• Gravitational lensing. It measures tiny distortions in galaxy shapes caused by gravity bending light — an effect that exposes invisible matter, including dark matter.

Neither works alone, Silvestri stresses. "A single galaxy cannot tell us much about dark energy or the laws of gravity. But billions of galaxies can." By studying patterns in galaxy positions and shapes, researchers can reconstruct the universe's history and test the physics behind it.

The scale of that task is its own problem. "Statistics is the key," Silvestri says. "That is why our colleague Guadalupe Cañas Herrera, a postdoctoral researcher at Leiden Observatory, has already started developing the tools needed to tackle this statistical challenge."

Is our understanding of gravity complete?

Beyond dark matter and dark energy, Euclid will probe an even more fundamental question. "Is our current understanding of gravity complete, or not?" Silvestri asks. She wants to use Euclid's data to test whether the laws of physics behave as expected on the largest scales — the cosmic scale.

Those tests will take time. The first results are expected in June 2027, the same date slated for the mission's next major milestone: combining observations into detailed cosmological analyses.

Astronomers have already published around 20 scientific papers describing the new data and first analyses, with dozens more in the pipeline. So while this release marks an important milestone, it's only the opening chapter of a six-year campaign.

"Euclid is revolutionizing the way we study our universe, providing unprecedented insights and setting a new standard for cosmology," Silvestri says. "Euclid's images are breathtaking. Producing images with such extraordinary detail across such an enormous area of the sky has required a truly remarkable effort."

Still, she tempers any sense of a finish line. "We are only at the beginning of our journey. The real scientific adventure starts now."