It all began with a hot, dense state we call the Big Bang. After 380,000 years, the universe cooled, and light separated from matter. This cosmic microwave background, like the very first chord, still fills space. Years later, Georges Lemaître suggested, and Edwin Hubble confirmed through observations, that galaxies are flying apart: this is the expansion of the universe. Still later, Saul Perlmutter studied distant supernova explosions and found that the expansion is not just happening but accelerating. The culprit was named dark energy. It makes up nearly 70% of all there is. What it is remains a mystery.

Light from the first glow travels for billions of years. Along the way, it encounters clumps of dark matter and galaxies. Their gravity slightly alters the energy of photons, like sound reflecting off walls. If the walls are built slowly, the tone shifts differently. Thus a gravitational echo is born. Scientists took two maps. One was a map of the ancient light from the Planck satellite. The other was a map of the locations of 600 million galaxies, based on infrared light from cosmic dust observed by the WISE telescope. Distances were determined via redshift — the stretching of light due to expansion. A special technique, akin to a fine musical ear, helped combine the maps and detect the faint echo amidst noise. Computer numerical simulations confirmed that the signal appears where theory predicts.

The result was 20% brighter than with old methods. This is the first direct indication. Dark energy truly hinders the formation of large structures — galaxy clusters. Now scientists want to slice the universe into time layers and listen to how the echo changed. The SPHEREx satellite will build multi-color maps. Perhaps we'll hear not just one note but a whole symphony, and understand whether dark energy is constant or has changed over time.