The universe expands by stretching space itself rather than moving into empty surroundings.

The concept of metric expansion was first proposed in the late 1920s by Georges Lemaître and Edwin Hubble. Hubble's landmark 1929 study used the 100-inch Hooker telescope at Mount Wilson Observatory to observe that the light from distant galaxies was 'redshifted.' This redshift occurs because the wavelengths of light stretch as the space they travel through expands.According to Einstein's General Theory of Relativity, space is a dynamic fabric rather than a static void. The Friedmann-Lemaître-Robertson-Walker (FLRW) metric provides the mathematical framework for this expansion, showing that the scale factor of the universe increases over time. This means that every point in the universe is moving away from every other point, with no central origin point for the expansion.Current measurements from the European Space Agency's Planck mission estimate the expansion rate, known as the Hubble Constant, at approximately 67.4 kilometers per second per megaparsec. This implies that for every 3.26 million light-years of distance, space expands by about 67 kilometers every second. Modern astrophysics suggests that dark energy, which makes up roughly 68 percent of the universe, is the primary force driving this accelerated stretching of the cosmic fabric.