When I began teaching in 1964, part of the required reading for my general studies science course were two articles published in Scientific American eight years previously, by two prominent physicists.
George Gamov, a principle architect of the Big Bang theory, made the case for a universe that began some billions of years ago as an explosion from an infinitely dense and infinitely small seed of energy.
Fred Hoyle, stalwart champion of the Steady State, took the stand for a universe with no beginning and no end, in which matter is continuously created in the space between the galaxies.
Both theories explained the outward rush of the galaxies discovered by Edwin Hubble and Milton Humason at Mount Wilson Observatory in the 1920s. Both theories had strengths and weaknesses. For example, the Big Bang successfully accounted for the known abundances of hydrogen and helium in the universe, but indicated a universe that was younger than the apparent ages of certain rocks on Earth. The Steady State theory avoided the stumbling block of a universe that seemed to come from nowhere, but replaced it with many little unexplained beginnings (those particles of matter appearing continuously from nothing).
However, the Big Bang theory made one prediction that was precisely testable: If the universe began in a blaze of luminosity, a degraded remnant of that radiation should still permeate the universe, and the spectral distribution of this microwave-frequency background radiation could be calculated.
Then, the very next year, 1965, entirely independently, two radio astronomers at Bell Labs in New Jersey, Robert Wilson and Arno Penzias, were trying to find the source of an annoying hiss in their microwave antenna that seemed to come equally from all parts of the sky. The hiss turned out to have precisely the characteristics predicted by the Big Bang cosmologists -- a knock-out blow to the Steady State.
The effect was breathtaking. What an exciting beginning to a long teaching career for me and my students, as spectators to history. For the first time, the human mind had constructed a creation story that could be tested empirically!