In 1923, Edwin Hubble showed that the "spiral nebulae" that were supposed to be within our galaxy actually are galaxies lying outside the Milky Way. He was then actually able to obtain the distances to some by studying Cepheid variable stars.

The following year, he showed that the Milky Way was just one of many galaxies in the universe. He and Milton Humason, using the newly completed 100 inch (2.5 m) reflector at Mt. Wilson, California, were the first to resolve individual stars in the Andromeda Galaxy (M 31), then to show that it was a galaxy, then to show that it was a galaxy by using the Cepheid variables to show that it was twenty-five times farther away than the farthest stars in our galaxy.

In 1925, Hubble invented a classification system for galaxies using their shape (spiral, elliptical, and irregular). Just four years later, Hubble presented evidence for the expansion of the universe:

1. He showed that the redshifts (the tendency for the light emitted to go towards the red end of the spectrum) discovered by Vesto M. Slipher, were telling us that the galaxies are speeding away at possibly thousands of miles per second.
2. Because they are going so fast, their light waves are stretched out, making them appear redder. He noticed that dimmer objects, thus objects farther away, had a larger redshift. (Picture raisins spreading farther and farther apart in a rising loaf of bread.)
3. Thus, Hubble's Law states that redshifts increase in proportion to their distance from us. This law enables modern astronomers to estimate distance in the universe. This law has also been verified many times over the last fifty years.

In the thirties, Hubble made a grave mistake in claiming that the galaxies are evenly distributed in space. To "prove" his point, Hubble took a large number of photographs of small regions of the sky. And, except for an area around the Milky Way which he called a zone of avoidance, he found galaxies in roughly equal numbers everywhere. Unfortunately for Hubble, other scientists disagreed.

Harlow Shapely and Adelaide Ames took large pictures of the Northern Hemisphere's sky. They noted large discrepancies in the concentration of galaxies. Clyde Tombaugh, who discovered Pluto in 1930, confirmed their conclusions, and in 1937 discovered that galaxies are arranged in clusters and superclusters.