Euclid's first major data release captures over 1 billion cosmic objects

For researchers, the practical change is the size and duration of the available sample, not a single completed discovery.

Why does Euclid observe from space?

Earth’s atmosphere no longer interferes with Euclid’s view of incoming light. From orbit, the telescope can record faint signals from distant galaxies, galaxy clusters and larger superclusters. Studying these immense structures can reveal how matter assembled across the cosmic web, a network of gas, dust and dark matter.

ESA has nicknamed Euclid the Dark Universe Detective because it targets dark matter, the unseen matter traced through gravity, and dark energy, the force accelerating the universe’s expansion. Together, scientists sometimes call these cosmic elements the dark universe.

Euclid Consortium member Barbara Sartoris of JL Lagrange Laboratory in Nice, France, said the telescope looks far back in time across huge areas of sky at once. That gives scientists unprecedented access to large samples of ancient galaxy clusters that were previously out of reach, she said.

In coming months, teams will examine how these giant structures grew and what they reveal about the dark universe.

Space-based viewing gives Euclid both a deep look into the past and broad coverage of the sky.

What can 15,000 strong gravitational lenses reveal?

The new catalogue lists 15,000 strong gravitational lensing candidates. Gravitational lensing follows from Einstein’s general relativity, introduced in 1915. In that framework, mass warps spacetime—the fabric of space—and gravity arises from that warping. Light normally travels in a straight line, but this curvature bends its path.

When a massive galaxy or galaxy cluster lies between a distant source and a telescope, the same light can take different routes and arrive at different times. The lens can greatly amplify the background source. The James Webb Space Telescope, known as JWST, has already used this effect to study galaxies in the early universe.

The Euclid catalogue gives researchers a route to weighing individual galaxies, including their invisible dark matter, and to examining the universe’s expansion rate, which reveals dark energy’s effect.

Natalie Lines of the University of Portsmouth in the U.K., an Euclid Consortium member, put the value plainly: “Strong lensing provides unique insight into the dark universe, allowing us to ‘weigh’ individual galaxies, including their invisible dark matter, and explore how the universe is expanding.”

Lines recalled that, more than 10 years ago, the team forecast Euclid would revolutionize strong-lens discovery and said that day had finally arrived.

The lens catalogue turns bent light into measurements of expansion and hidden mass.

How much more will Euclid observe?

The observations behind DR1-Foundation run from spring 2024 to spring 2025 and cover 14% of the sky area targeted for Euclid’s full six-year survey. The release therefore represents the survey’s opening stage, not its endpoint.

Pettorino said the package contains raw data, calibrated images, catalogues and spectra across a huge sky area, with enough statistics to uncover even rare objects. The material will support mapping of the universe’s large-scale structure in unprecedented detail.

On the evidence of this release, the strongest conclusion concerns research capacity rather than a final answer: Euclid has assembled a map-making dataset, while many discoveries remain ahead. “So much discovery is yet to come,” Pettorino said.

The release supplies the evidence base for later discoveries rather than a finished account of the dark universe.