It’s tough to believe now, but the Big Bang theory was very controversial back in the day. The definitive prediction of the theory, a radio echo of the “bang” part, wasn’t spotted until thirty years after the theory was hatched. But once it was, ticking off the evidence in favor of the Big Bang became a bore.

Each one of these predictions, like a uniformly expanding Universe whose expansion rate was faster in the past, a solid prediction for the relative abundances of the light elements hydrogen, helium-4, deuterium, helium-3 and lithium, and most famously, the structure and properties of galaxy clusters and filaments on the largest scales, and the existence of the leftover glow from the Big Bang — the cosmic microwave background — has been borne out over time.

All of that I knew, thanks to Simon Singh. But this was new:

But there was another prediction we haven’t talked about much, because it was thought  to be untestable.  …  Hypothesized in 1930 to account for missing energies in some radioactive decays, neutrinos (and antineutrinos) were first detected in the 1950s around nuclear reactors, and later from the Sun, from supernovae and from other cosmic sources. […]

the Big Bang makes a very explicit prediction: There should be a cosmic neutrino background (CNB) that is exactly (4/11)^(1/3)  of the cosmic microwave background (CMB) temperature. That comes out to ~1.95 K for the CNB, or energies-per-particle in the ~100–200 micro-eV range. This is a tall order  for our detectors, because the lowest-energy  neutrino we’ve ever seen is in the mega-eV  range.

We haven’t measured the temperature of the CNB (yet), but we do have indirect confirmation it exists. Ethan Siegel does an excellent job of explaining the details, so I’ll turn the floor over to him.