the.com/cosmic microwave background

means The faint, uniform glow of radiation left over from the hot early universe, now stretched by cosmic expansion into microwaves that fill all of space.

from Predicted in the 1940s from Big Bang theory, the CMB was found by accident in 1964 when two radio astronomers trying to eliminate noise from their antenna in New Jersey kept picking up a persistent hiss — even after they scraped out what they called 'white dielectric material' (pigeon droppings) and evicted the birds responsible. The hiss remained: it was radiation from roughly 380,000 years after the Big Bang, the moment the universe cooled enough for light to finally travel freely. The name itself is plain physics-speak — 'cosmic' for its origin, 'microwave' for the wavelength it now occupies after billions of years of stretching, and 'background' because it hums behind everything else in the sky.