Why astronomers take the idea seriously

The evidence

Three strands come up again and again in the pages we checked: galaxies receding, a leftover glow, and light-element amounts that match what the theory predicts.

Edwin Hubble correlated these observations in mathematical form to provide evidence that the universe is expanding. Britannica We know this, he says, because we see galaxies rushing away from each other, not from a central point, and because leftover heat uniformly fills the universe. NASA

The discovery of the 2.7 K cosmic microwave background radiation in 1965 by Arno Penzias and Robert Wilson was convincing evidence that the universe originated 13.8 billion years ago from a very dense and hot state in the big bang. Britannica The cosmic microwave background, Britannica says, is electromagnetic radiation filling the universe that is a residual effect of the big bang 13.8 billion years ago. Britannica

After a few seconds, the universe cooled enough to allow the formation of certain nuclei. The theory predicts that definite amounts of hydrogen, helium, and lithium were produced. Their abundances agree with what is observed today. Britannica

Maps of the CMB provide a precise measurement of the geometry of the Universe, and show that it is “flat,” governed by Euclidean geometry on cosmic scales. NASA It nailed down the curvature of space to within 0.4% of “flat” Euclidean. NASA

We observe that same light today, stretched by the expansion of the universe to a cold 2.7K glow. NASA

None of this means every detail is settled. The notes on this site are there because good sources still disagree about some of the wording — the age, the timing of the afterglow, and what triggered inflation among them.