It only took three pieces of evidence, together, to completely revolutionize our picture of the Universe in the early 20th century. We had previously assumed that the Universe was static and unchanging, and that the entirety of existence was contained within the Milky Way. The spiral and elliptical nebulae in the sky were assumed to be objects — perhaps newly forming protostars — within our own galaxy. And yet, by the late 1920s, the combined facts of:

• Einstein’s General Relativity superseding Newton’s universal gravitation as our theory of gravity,
• individual stars being observed in those spiral and elliptical nebulae, allowing us to confirm their extragalactic nature and to compute the distance to them,
• and the observational shifting of atomic lines towards either redder or bluer wavelengths from spiral and elliptical galaxies,

overturned that picture entirely. The Universe, on the largest of cosmic scales, wasn’t static and unchanging at all, but rather was dynamically expanding.