[antognini]

One of the other major pieces of evidence for the existence of dark energy is the observation of baryon acoustic oscillations. The basic idea is that as the universe was forming, there were concentrations of dark matter. Standing sound waves of photons and baryons (ordinary matter) formed around these concentrations. After decoupling, the photons streamed away, but the baryons remained and eventually formed galaxies. This results in a characteristic pattern seen on the sky, where there are clusters of galaxies observed, and then rings of galaxies around them.

It is possible to calculate the physical size of these rings from first principles. Since you observe how big these rings appear on the sky, you can figure out how far away they are, and from this determine the expansion history of the universe.

[vorg]

> as the universe was forming, there were concentrations of dark matter. Standing sound waves of photons and baryons (ordinary matter) formed around these concentrations

You make it sound like "dark matter" is normal matter and the "ordinary matter" is the abnormal stuff, since ordinary matter formed afterwards and has more complex interactions in that it supplements gravitation with electromagnetism and what not. And of course there's far more dark matter than ordinary.