Funny you should ask. Euclid specifically measures the ellipticity of billions of galaxies. Because the light traverses regions of lower and higher density (ma

Funny you should ask. Euclid specifically measures the ellipticity of billions of galaxies. Because the light traverses regions of lower and higher density (made up mostly of dark matter), we see a gravitational lensing effect. Sources of gravity distort the image behind the "lens". The article describes a strong lensing effect, which is pretty self-evident. By analyzing correlations of galaxy ellipticities we can measure an effect which is only apparent as a statistical effect. You'd expect no intrinsic correlation of the apparent ellipticity between galaxies (because they can be millions of light-years apart), but this so called weak lensing effect does create a correlation. I basically think of it as observing perfect ellipses through slightly obscure glass and deducing from that the exact shape of the glass. Thanks to Euclid's data, we will be able to create a sort of 3D map of the total matter distribution (dark and baryonic), which will in turn tell us how the universe evolved better than any other probe. We hope to learn more about dark energy in this way.