Edwin Hubble discovered in the 1920s that the universe is expanding. He noticed that galaxies are receding from us and the farther a galaxy is the faster it is moving away. The expansion rate is known as the Hubble constant and is currently estimated at 70 km/s/Mpc. This means that for every Megaparsec (3.26 million light-years), galaxies are moving away at a speed of 70 km/s.