In the previous post, we saw how the first nuclei formed.
The universe was then made mostly of hydrogen nuclei, helium nuclei, electrons, and light. But these were not yet atoms. The temperature was still too high for electrons to remain bound to nuclei for long.
Any atom that tried to form was quickly broken apart again.
So the universe remained a plasma: a mixture of free charged particles and light. It was not empty, and it was not dark. It was full of light. But that light could not travel freely.
light interacts efficiently with electric charge. As long as electrons were free, light was constantly scattered. It could move for a while, but soon it would meet another electron and scatter again, and again.
In this state, matter and radiation were tightly linked. The universe was more like a glowing fog than a transparent space.
This changed as the expansion continued.
As the temperature fell to a few thousand degrees, the light became less energetic. Eventually, electrons could settle into bound states around nuclei without being immediately knocked away. Free electrons disappeared into neutral hydrogen and helium atoms.
That small change altered the behavior of light.
A neutral atom does not scatter light as efficiently as a free charged particle does. This does not mean that neutral matter stops interacting with light altogether. Neutral atoms can still absorb, emit, and scatter radiation, and many familiar objects around us are neutral without being transparent. The difference is that the universe was no longer filled everywhere with free electrons, and free electrons are especially effective at keeping light trapped in a constant cycle of scattering.
Once most electrons were bound into atoms, that cycle largely stopped. Light could travel through space with far fewer interruptions.
The universe became transparent.
The universe was now filled mostly with neutral hydrogen and helium, together with light that had been released from its constant scattering.
That light is still present today.
As the universe expanded, its wavelength was stretched. What began as hot radiation became colder and less energetic. Today we observe it as the cosmic microwave background.
It is the oldest light we can see directly.
The cosmic microwave background is not light from stars or galaxies. It comes from an earlier time, before stars existed. It is the cooled remnant of the hot plasma that once filled the universe.
When we observe it, we are seeing the snapshot of the universe during the stage at which photons last scattered from free electrons before traveling almost freely across space.
This is why the formation of atoms matters.
It did not create stars, planets, or heavy elements. It simply allowed light to separate from matter.
After that separation, matter could continue gathering under gravity, while light carried away a record of the earlier state.
In other words, this marks the moment when the universe stopped being opaque.
It is the reason the early universe can still be seen.
See also: What came before atoms?