By analyzing color indices — the ratio of brightness in different spectral bands in images from the James Webb Space Telescope, astronomers have identified galaxies whose light cuts off sharply in the ultraviolet. This cutoff is caused by hydrogen fog — neutral hydrogen that, at a certain distance (corresponding to redshift z~10.6), efficiently absorbs ultraviolet photons, leaving a characteristic spectral step. This method allowed them to select the oldest stellar systems. They then compared the positions of these galaxies with the distribution of invisible clouds of dark matter — a mysterious substance that reveals itself only through gravity, whose influence on the rotation of galaxies was first measured in detail by Vera Rubin.

It turned out that in the era when the universe was only 3% of its current age (redshift z~10.6), a typical dark matter halo had a mass of about 12 billion solar masses — tens of times lighter than at later times (by z~5.5, the mass had grown to 280 billion solar masses). Paradoxically, it was in these lightweight cradles that galaxies showed increased crowding: they were packed more densely, had almost no satellites, but often experienced violent mergers — gravitational collisions that lead to rapid growth. Thus, like moving from frugal sketches to massive foundations, the invisible framework of the universe grew: miniature clouds of dark matter gradually merged, forming ever more imposing structures.