Everything,
Receding.

In 1923, Edwin Hubble photographed a Cepheid variable star inside the Andromeda nebula and measured its distance: 900,000 light-years. The Milky Way is only 100,000 across. Andromeda was not a cloud inside our galaxy. It was a galaxy of its own. The universe got a billion times bigger overnight.

Six years later, in 1929, he plotted the distances of forty-six galaxies against their redshifts and found a straight line. The farther a galaxy lay, the faster it was receding. v = H · d. Run the clock backward and everything was once together. The Big Bang follows from that line.

The deeper truth is subtle: the galaxies are not moving through space. Space itself is stretching, and the galaxies — like raisins in rising dough — are carried along. There is no point of origin. No center. From any galaxy you choose as home, the universe expands away from you.

v = H₀ · d

The current best measurement of H₀ — the Hubble constant — is roughly 70 km/s per megaparsec. Or rather, two best measurements: one from supernovae says 73, one from the cosmic microwave background says 67. The four-unit gap is called the Hubble tension, and it is, at present, the most embarrassing problem in cosmology.