Cosmic microwave background radiation
Cosmic background microwave radiation (CMB radiation) is radiation in the microwave part of the electromagnetic spectrum, which comes from all directions in outer space. It is known to come from our earliest infant universe. Since the universe is very large, and the speed of light is constant, we know that when the CMB light arrives from the infant universe, it arrives as the oldest signal that we can detect.
During the Big Bang, a lot of high-energy radiation was created. Then, the universe became bigger and colder. Therefore, the high-energy photons lost most of their original energy. Now, as a result, that radiation is in the microwave part of the electromagnetic spectrum (the microwave part has quite low energy). The cosmic microwave background is the radiation that has been traveling without hitting anything ever since the time the universe became transparent, about 380,000 years after the Big Bang.
Later data is based on the Planck spacecraft operated by the European Space Agency (ESA). It was designed to observe differences in the cosmic microwave background (CMB) at microwave and infra-red frequencies, with high sensitivity and small angular resolution. The spacecraft has finished its work, but researchers are still analyzing the data. The main interest is that there is:
"an asymmetry in the average temperatures on opposite hemispheres of the sky. This runs counter to the prediction made by the standard model that the Universe should be broadly similar in any direction we look. Furthermore, a cold spot extends over a patch of sky that is much larger than expected." No explanation for this is known.