Discovery of cosmic microwave background radiation
The discovery of cosmic microwave background radiation constitutes a major development in modern physical cosmology. In 1964, American physicist Arno Allan Penzias and radio-astronomer Robert Woodrow Wilson discovered the cosmic microwave background (CMB), estimating its temperature as 3.5 K, as they experimented with the Holmdel Horn Antenna.
Source: Wikipedia — Discovery of cosmic microwave background radiation (CC BY-SA 4.0)