He discovered the existence of galaxies outside our own, and classified them according to their shape. His theory that the universe is expanding is now generally accepted.

His data on the speed at which galaxies were receding (based on their redshifts) were used to determine the portion of the universe that we can never come to know, the radius of which is called the Hubble radius. Beyond this limit, any matter will be travelling at the speed of light, so communication with it will never be possible. The ratio of the velocity of galactic recession to distance has been named the Hubble constant.

Hubble discovered Cepheid variable stars in the Andromeda galaxy in 1924, proving it to lie far beyond our own Galaxy. In 1925 he introduced the classification of galaxies as spirals, barred spirals, and ellipticals. In 1929 he announced Hubble's law, stating that the galaxies are moving apart at a rate that increases with their distance from each other.

Hubble was born in Marshfield, Missouri, and studied at the University of Chicago, Illinois, graduating in 1910, and then at Oxford University, where he gained a degree in law in 1912. He was also an athlete and a heavyweight boxer. He returned to America 1913 and briefly practised law in Louisville, Kentucky, but his real interest was in astronomy, and he went to the Yerkes Observatory, Williams Bay, Wisconsin, as a graduate student in astronomy in 1914. He was awarded a PhD in 1917 for a thesis on the photography of faint nebulae. After serving with the American Expeditionary Force in World War I, he joined the staff at Mount Wilson Observatory, near Pasadena, California, in 1919, and carried out research on galactic and extragalactic nebulae. During World War II he was chief of ballistics and director of the Supersonic Wind Tunnel Laboratory at the Aberdeen Proving Ground, Maryland. Afterwards he returned to what was to become the Mount Wilson and Palomar Observatories, and was one of the first to use the new 200-inch Hale Telescope that was installed in 1948.

Nearly all Hubble's work related to nebulae, which he was the first to show were extragalactic, that is, located outside our own Galaxy. It has been said that Hubble opened up the observable region of the universe in the same way that the Italian astronomer and physicist Galileo opened up the Solar System in the seventeenth century, and British astronomers William and John Herschel opened up the Milky Way in the eighteenth and nineteenth centuries. He gave an account of some of his researches in The Realm of the Nebulae (1936).