The night sky looks black, but if you remove the stars and accumulate light for a long time, a faint glow emerges — the summed light of all galaxies born over 13.8 billion years. Astronomers call it the extragalactic background light. Twenty years ago, the CIBER rocket experiment, in 220 seconds of observations, found that this background in the near-infrared flickers more than known galaxies should produce. Where the excess came from was a mystery.

The breakthrough came when CIBER maps were compared not with themselves but with galaxy catalogs. This cross-examination of sky and galaxies, cross-correlation, shows where exactly the light concentrates. Galaxies were sorted into layers, like in tomography, based on distance. It turned out that the excess brightness is concentrated in nearby layers — at distances up to 6 billion light-years. It is not distant quasars or the first stars, but ordinary galaxies and their invisible halos.

The metaphor of cross-examination holds to the end: the sky 'answers' the catalog's question, and in the answer, you hear the voice of invisible structures. Galaxy light does not simply add up: it is drawn toward dark matter clumps like fog around streetlights. This fog turned out to be brighter than thought.

Previously, the excess fluctuations were attributed to calibration errors or unknown sources. Now it is clear: a significant part comes from low-redshift large-scale structure. This changes the interpretation of the background light and will help future missions CIBER-2 and SPHEREx, which will map the entire sky with much higher detail.

The cross-examination of sky and galaxies continues. Each new catalog is another question, and the Universe answers in ever more detail.