This is a cool article from BBC News but the theory behind it is poorly explained. So I investigated somewhat to catch myself up on the topic.
Essentially, the article discusses how scientists called the BOSS (the Baryon Oscillation Spectroscopic Survey) have measured variations in the density of baryons in the universe. Because of events in the early universe, this variation has a particular specific size which can then be used to calibrate distances within the universe to great accuracy.
To get the point, we have to go all the way back to the beginning of the universe...
This is how I understand it.
Big bang goes boom, so to speak.
Everything in the universe is a hot, expanding soup. Although this soup is very uniform, it isn't perfectly uniform. There are tiny differences in density of this soup on the scale of 10 parts per million. It is currently thought that all variation in density of matter in the universe today (compare a galaxy to the relatively empty space between galaxies) came from these 10 parts per million variations, which can be pictured in the Wilkinson Microwave Anisotropy Probe map which many have seen but few know the name of ("That Universe Map Thing" is how I thought of it). Overly dense regions would be "seeds" that would attract more matter, while the slightly less dense regions would lose matter to those seeds. Tiny variations in density plus gravity plus time ultimately give you the rather lumpy universe we now live in where the largely uniform soup has lumps we call stars and galaxies and cats.
Now let's look more closely at that early soup of a universe with only tiny lumps. It is made up of some of the most fundamental "stuff," mainly lots of dark matter (which we still know very little about), baryons (basically think of it as what makes up matter as we know it in our daily lives plus a bit of more exotic stuff), and photons (light). It is so dense that the baryons and photons strongly interact, something they don't do much of today.
The tiny bits of overdense matter, as I said, attract more matter. The energy generated by the interaction creates an outward push. Inward gravity affects everything, including dark matter, but the outward push only pushes out the interacting baryons and photons, creating a kind of pressure wave of baryons and photons moving away from the central "seed" of density as it grows.
As the universe expands (mostly uniformly) it cools, but at the overdense seeds of matter the growing lumps send out these pressure waves within the otherwise fairly uniform soup. I think that gives a reasonably accurate picture of what was going on early in the universe.
Then, something big happened, maybe the biggest that ever happened since the Big Bang itself (of course with the exception of the invention of sliced bread, the standard scientific measure of "big" events). Around 379,000 years after the big bang boomed, the universe (still fairly uniform except for the growing seeds of matter) cooled enough that the strong interaction between photons and baryons, light and ordinary matter, suddenly was broken. Almost uniformly throughout the young and expanding universe, photons shot away from the baryons as fast as they can (which is, by definition, at the speed of light) and baryons kind of kept going at their much more sedate pace.
This singular event 379,000 years after the big bang created ripples of baryons around each growing seed of stuff in the universe. This isn't trivial. What it does is create a new structure to the universe where there is a fairly uniform sea of photons (this is background microwave radiation that fills the universe, representing the outflow of photons 379,000 years after the big bang when photons and baryons stopped interacting so tightly), within which are growing seeds of dense matter, surrounded by ripples of thinner matter. The radius of these ripples at the moment the photons flew away from the baryons can be simply calculated.
Let gravity do its work over billions of years and you get denser clusters of galaxies around those original seeds of denser matter, thinner clusters of galaxies where the expanding ripples are, and a more uniform background radiation in between. That is, in an oversimplified manner, what we have today and how it formed. Unless of course you don't believe in science, in which case photons and baryons still haven't decoupled since the universe is only 6000 years old, not long enough to cool down enough for decoupling. In which case you aren't around yet to read this.
Now, using super cool and big telescopes (say the 2.5m Sloan Foundation Telescope in New Mexico), scientists (like the folks of BOSS) have been able to measure the radius of the ripples today, after billions of years of expansion. And they turn out to be about 490 million light years across. This is a very accurate way to measure distances in any region of space we can visualize, giving us a more accurate than ever measure of the size of the universe, and to measure more accurately than ever the expansion of the universe since the time of decoupling of photons and baryons (details of which were announced at the 223rd American Astronomical Society in Washington DC and can be found as a preprint here).
So there you have it. The latest in cosmology and how we got here from the early universe. Naturally there are more details I leave out (like the origin of the baryons and other stuff, which came even earlier than the events I describe), are yet to be discovered (like more info about the properties of dark matter), or may never be explained (like the physics describing the formation of cats).