The hunting season for dark matter and more is open — the first data package from the Euclid telescope has been published

On March 18, 2025, the European Space Agency released the first batch of observations from the Euclid space observatory, nicknamed the dark matter hunter. The data includes three deep sky surveys conducted in the first week of observations, covering a total area of ​​63.1 square degrees. Scientists called them a “gold mine” for starting the hunt for the secrets of the Universe, including the main ones – the search for dark matter and solving the mystery of dark energy.

The yellow blobs in the Milky Way data are Euclid’s first deep surveys. Below are photos of the surveys. Image credit: ESA

Launched in July 2023, Euclid began science operations in February 2024. The first publication included data collected during the first week of observations: three deep surveys of small patches of sky totaling 63.1 square degrees. That’s just 0.4% of the full survey that will cover a third of the sky and run until 2030. But even this modest data will be enough to make many important discoveries in astronomy.

Deep surveys of the first three patches of sky – two in the southern part of our galaxy and one in the northern – included 380,000 classified galaxies, 500 new gravitational lens candidates, and many other cosmic objects such as galaxy clusters and active galactic nuclei.

For the first time, artificial intelligence was used to find the most promising objects for further observation, which dramatically reduced the time for selecting candidates and, accordingly, the time for conducting scientific work. The candidates selected by AI were then transferred to citizen scientists – volunteers who voluntarily classified the objects, thereby saving the time and resources of professional researchers.

The first elements of the future Euclid atlas have already served as the basis for the publication of dozens of scientific papers, including a study dedicated to the discovery of a perfect Einstein ring. This phenomenon occurs as a result of gravitational lensing, when a distant object and a massive galaxy or cluster of galaxies acting as a gravitational lens are in the same line with the observer (in this case, Euclid).

«Euclid was the first space telescope to routinely detect gravitational lenses. Almost all of the 500 gravitational lenses it found were new. By the end of its observations, the observatory is expected to have found 100,000 gravitational lenses—100 times more than are known today.

The observatory peers into the depths of the Universe 10.5 billion years back in time. At this entire distance, it reveals features of the structure of galaxies. The shape or morphology of galaxies — the location and type of arms, types of star clusters, and other details — allow us to estimate the distribution of dark matter around each of them. At the same time, clusters and the location of galaxies in the cosmic web system are determined by the external influence of dark matter.

Both of these factors, the internal and the external, shape the appearance of galaxies and their mutual arrangement. Today we cannot say with certainty what dark matter is. However, the data meticulously collected by Euclid promises to help solve this mystery. The invisible will manifest itself through its global impact on visible matter.

A fragment of one of the sections magnified 70 times

A similar situation is developing with dark energy. Some force is causing galaxies unbound by gravity to fly away from each other at an accelerating rate. What exactly is pushing them apart remains a mystery. Euclid will also help to put the tightest constraints on this invisible force, by creating the most accurate dataset of many galaxies at great depth.

An example of a series of classified galaxies from the first survey

Work with the first data from the observatory has already begun. The first year of Euclid’s work is expected to be published in 2026, and will include 2 PB of data. Today’s review may seem modest against this background – only 35 TB, but this is information for only one week of observations. Euclid will pass over each of the already passed areas of the sky 30 to 50 times, each time increasing the resolution and improving the quality of the images. By 2030, this will be the most complete and detailed catalog of galaxies, which has no equal and probably will not exist for a long time.