Stars group together to form galaxies. Galaxies form galaxy clusters. These form superclusters, between which vast, largely empty regions extend, the voids. All superclusters are connected by a honeycomb-like basic structure, the “cosmic web”, which consists of filamentary gas structures of hydrogen. That these filaments must exist has been known for some time. On the one hand, they are known from simulations based on theories of the structure of the universe, which predict such a basic structure. On the other hand, they become visible when energetic quasars illuminate them like car headlights illuminating the nebula.

However, the regions thus detected are poorly representative of the entire network of filaments in which most galaxies, including our own, were born. Direct observation of the faint light emitted by the gas that forms the filaments has been considered the holy grail among astronomers. It has now been found by an international team led by Roland Bacon, CNRS researcher at the Centre de Recherche Astrophysique de Lyon.

The team realized this step with the help of ESO’s Very Large Telescope (VLT). It is equipped with the MUSE instrument, which is coupled to the telescope’s adaptive optics. Together, the two instruments form one of the most powerful optical systems in the world. For more than 140 hours, researchers focused it on a single region of the sky. It is part of Hubble’s Ultra-Deep Field, which was previously the most comprehensive image of the cosmos ever taken. However, the new image has now surpassed Hubble, as 40% of the galaxies discovered by MUSE have no counterpart in Hubble images.

After meticulous planning, it took eight months to complete this extraordinary observing campaign. A year of data analysis followed, revealing light from the hydrogen filaments for the first time. The images show several filaments as they appeared one to two billion years after the Big Bang, a key period for understanding how galaxies formed from the gas in the cosmic web. The biggest surprise for the team, however, was when the simulations showed that the light emitted by the waterfall came from a previously invisible population of billions of dwarf galaxies. Although these galaxies are too faint to be detected individually with current instruments, their existence will have major consequences for galaxy formation models, with implications that scientists are just beginning to explore.