Roughly 380,000 years after the Big Bang, about 13.7 billion years ago, matter (mostly hydrogen) cooled enough for neutral atoms to form, and light was able to traverse space freely. That light, the cosmic microwave background radiation (CMBR), comes to us from every direction in the sky, uniform except for faint ripples and bumps at brightness levels of only a few part in one hundred thousand, the seeds of future structures like galaxies.
Source:
https://phys.org/news/2020-02-cosmic-microwave-background.html