It was Einstein who first hypothesized that the universe is homogeneous and looks the same in all directions (isotropic). A century later this hypothesis still holds, and yet we do not know exactly why this should be the case.
By describing the universe as homogeneous and isotropic, Einstein did not mean to say, for example, that a small town in India will look the same as a small town Indiana. Rather, Einstein’s hypothesis refers to the fact that the patterns that millions of galaxies make on the sky look similar in all directions. So the pattern that one million galaxies makes on the sky looks the same as the pattern that a different one million galaxies will look like in a different direction of the sky.
How can this be?
Astronomers do not know the answer for certain. The best guess we have is that at one point in history the universe really did have the properties of a single small town. We think that early on in the universe’s history, and long before the first stars and planets ever existed, a small portion of the then small universe suddenly exploded. The event would have been dramatic, but importantly would not have had the property of destroying any matter. Instead, a small part of the universe basically just ‘ballooned’ out.
A short time later this explosive expansion stopped and only the more ‘gentle’ situation of universal expansion discovered by Edwin Hubble remained. We call this short, explosive expansion phase inflation. As a result of inflation, galaxies that were neighbors beforehand and shared similar physical attributes continued to share the same physical attributes after inflation even though they are now on separated by vast distances.
The race to see if inflation is the correct explanation to support Einstein’s hypothesis is one of the most active fields of astrophysics today.
Space.com, March 12, 2015 - Did Cosmic Inflation Really Jump-Start the Universe? (Kavli Hangout)
International Business Times, Sept. 23, 2014 - Space Dust Findings Raise Doubts Over Cosmic Inflation 'Proof'
Nature World News, Jan 31, 2014 - Update: BICEP2's Proof of Big Bang Officially Foiled by Dust
Nature.com, May 14, 2014 - Cosmology: First light
The Paragraph, Sept. 22, 2014 - Emerson and Cosmic Inflation Editor's note: this post has several links to articles on inflation, the Big Bang, and related topics.