It seems that you're referring to the discovery of the Cosmic Microwave Background (CMB) radiation, which is indeed strong evidence for the Big Bang theory and the expansion of the universe.
The CMB is a relic radiation that fills the universe and is a remnant from the early hot phases of the universe just after the Big Bang. Its temperature is approximately 2.7 Kelvin, which is a remnant of the thermal radiation emitted when the universe cooled enough for atoms to form and light to travel freely.
By observing the CMB today, astronomers can see what the universe looked like approximately 380,000 years after the Big Bang, giving insight into the conditions and composition of the early universe. This observation supports the theory of cosmic expansion, as the CMB shows slight fluctuations that indicate the density variations that led to the formation of large-scale structures in the universe.
In summary, the accidental discovery of the CMB provides compelling evidence for the Big Bang theory and the ongoing expansion of the universe.