The Cosmic Microwave Background as detected by WMAP satellite: The red region is where matter is dense and the blue region is the voids.
But not for another 380000 years does much to our soup of atomic nuclei because the universe is still hot enough for the electrons to stay away from the nucleus flying freely with the photons. Until the temperature of the universe dropped to about 3000 degrees Kelvin, the electrons start to combine with the nucleus forming the first stable atoms and this process will release visible light photons that bath our universe. As the space-time continue to expand, the photons slowly lose its energy and changes its frequency from visible light towards infrared and now microwaves.
When astrophysicist now look into every part of the night time sky with microwave detectors, they find an unmistakable fingerprint of 2.73 degree Kelvin microwave photons whose patterns in the sky retains the memory of the distribution of matter just after atoms were formed.