Imagine looking up at the night sky. With our own eyes we can see at most a few thousand of the nearest stars to us. Now consider looking through a large telescope with a very large field of view. Through such an instrument millions of objects come into view, with most of these objects being galaxies, not stars.
Interestingly, galaxies are not scattered randomly about the sky, as one might expect. Rather, they trace out a structure that looks a bit like a 3D spider web, called by astronomers the “cosmic web.” Fair enough. The story gets more interesting though when we find out that the mean separation between galaxies, equating roughly to the mean separation between the threads of a spider web, was set early on in the universe’s history.
When the universe was only about 370,000 years old, various sound waves that traversed its extent were frozen into place by changing physical conditions. Astronomers maintain that events that happened at this time established the mean distance between the galaxies long before the first galaxy would ever form. These so-called Baryon Acoustic Oscillations (BAOs), named "acoustic" for their sound wave origin, refer to the physical signature of these ancient sound waves expressed in the universe today.
Put another way, just as archaeologists disinter dinosaur bones to learn about human history, astronomers measure the mean distance between galaxies to learn about the distant and early history of the universe. This work is carried out by the Sloan Digital Sky Survey (SDSS) Extended Baryon Oscillation Spectroscopic Survey (eBOSS), a large program which offers free public access to all of its data sets worldwide. If time allows, try visiting the website at legacysurvey.org, enter in your favorite coordinates in the upper right, and enjoy exploring the distant universe.