The James Webb Space Telescope looks back in time and sees the universe when it was less than a billion years old. In this young universe, galaxies already shine, and at their centers sit supermassive black holes — the ones that will later grow into giants. How did they manage to gain mass so quickly? To understand this, astronomers measured how galaxies and black holes cluster in space.
Galaxies with bright oxygen emission (visible even at vast distances) are packed into dense clumps: the average distance between neighbors is about 4.7 megaparsecs (roughly 15 million light-years, 300 times the diameter of our Milky Way). Black holes reside in even tighter and more massive "homes": their host galaxies are on average 15 times heavier than typical oxygen-emitting galaxies. This difference suggests that black holes are born only in the largest clumps of matter.
But the most surprising part is how briefly black holes remain bright. Calculations show that the active phase, when a black hole devours matter and shines like a quasar, lasts less than three million years. That's almost nothing compared to the time needed for growth. This means black holes gain almost all their mass in a "quiet" mode — without bright emission, unnoticed by telescopes. The early universe grew its giants in the dark.
The oxygen used to find these galaxies didn't appear right after the Big Bang: it was synthesized by the first stars, which exploded as supernovae. So the bright oxygen emission is a trace of stars that have already lived and died.
In Arthur C. Clarke's novel "The City and the Stars," a civilization hides from the outside world, and here black holes hide their growth from observers — only on a cosmic scale.