Empirical

The whole idea that space-time is expanding was first noticed asa mathematical consequence of general relativity by Georges Lemaitre in 1927, when he solved Einstein’s equation and found a solution for the metric showing that distances grow with time. It became clear two years later that the universe we live in is governed by such a solution when astronomer Edwin Hubble noticed that galaxies appeared to be racing away from each other. Lemaitre’s work provided a theoretical explanation for this empirical data: the yardstick for measuring cosmic distance was itself changing with time.
–New Scientist