31/07/2026
A CENTURY AGO, WE THOUGHT THE UNIVERSE WAS ALMOST JUST ONE GALAXY.
It is difficult to imagine today, but just over a century ago, astronomers were still trying to determine whether the Milky Way was the entire universe or merely one part of something vastly larger.
In the early 20th century, mysterious objects known as “spiral nebulae” were visible in telescopes, but their true nature was unknown. Were they clouds of gas and stars located somewhere inside the Milky Way—or were they enormous, distant systems comparable to our own galaxy?
This question became famous during the Shapley–Curtis Great Debate of 1920. Harlow Shapley argued for a very large Milky Way in which the spiral nebulae were not separate galaxies, while Heber Curtis argued that the spiral nebulae could be independent galaxies far beyond the Milky Way.
Then came one of the most transformative developments in the history of astronomy. In the 1920s, observations by Edwin Hubble provided compelling evidence that some of these mysterious spiral nebulae were actually distant galaxies. The implication was enormous: the Milky Way was not the universe—it was only one galaxy among countless others.
Today, NASA estimates that the Milky Way’s stellar disk spans more than 100,000 light-years, while the observable universe extends roughly 93 billion light-years across.
And the numbers become almost impossible to comprehend. NASA says the observable universe contains at least 100 billion galaxies, with each galaxy potentially containing millions, billions, or even trillions of stars.
In barely a century, humanity’s cosmic perspective changed dramatically—from imagining the Milky Way as the great island universe to understanding that it is just one member of a vast cosmic web of galaxies, clusters, filaments and enormous voids.
And perhaps the most humbling part is this: the observable universe is not necessarily the entire universe. We can only observe the region from which light has had time to reach us. What exists beyond that cosmic horizon remains unknown.
Our understanding of the cosmos has expanded from one galaxy to a universe filled with at least hundreds of billions of galaxies—and possibly something far larger beyond what we can ever observe.
The universe didn’t suddenly become bigger.
Our understanding of it did.
Source: NASA Science; NASA/IPAC Astronomy Picture of the Day — historical records of the Shapley–Curtis Great Debate.
