Dark matter makes up roughly 27 percent of the universe’s total energy content, according to the best available cosmological models. We know it exists because of the gravitational effects it has on visible matter. We have never detected it directly. We do not know what it is.
The Euclid space telescope, launched by the European Space Agency in 2023, is designed to map the large-scale structure of the universe in enough detail to constrain the properties of dark matter and dark energy. Its first major data release, in May 2024, delivered the most precise three-dimensional map of dark matter ever produced.
How the map works
Dark matter does not emit or absorb light. You cannot see it. But it bends light through gravitational lensing. When light from distant galaxies passes near concentrations of dark matter, the images of those galaxies are distorted in subtle ways. By measuring these distortions across millions of galaxies, Euclid reconstructs where the dark matter must be.
The first release covered a small fraction of Euclid’s planned sky survey, but even that sliver contained roughly 1.5 million galaxies and provided cosmological measurements competitive with results from much longer surveys.
What the data shows
The dark matter distribution mapped by Euclid matches the predictions of the standard cosmological model, called Lambda-CDM, reasonably well. But there are hints of tension, regions where the clumping of matter is slightly different from what the model predicts. These discrepancies could be statistical noise, systematic measurement errors, or early signals that the standard model needs revision.
One finding that attracted attention was the measurement of a parameter called sigma-8, which describes how clumpy matter is on large scales. Euclid’s measurement is slightly lower than what the cosmic microwave background predicts. This could indicate that the standard model is incomplete, that dark matter has properties not currently included in the model, or that both measurements have subtle errors.
What comes next
Euclid’s planned six-year survey will cover 15,000 square degrees of sky, about one third of the entire sky. The first data release covered roughly 12 square degrees. What the telescope has shown so far is that it works, its measurements are competitive with years of ground-based surveys, and the universe may have some surprises left in its large-scale structure.
That last point is the most interesting. We do not know what dark matter is. Euclid is the most powerful tool ever built for studying its distribution. The discrepancies in the first data release might be nothing. They might be the first crack in a model that has held for decades.