Bubbles, Baubles, Bright Shiny Galaxies
Back in the early days, when the Universe and I were young, the Universe (and I) were a hot dense plasma, with most of the mass and energy tied up in dark matter. A slight amount of clumping existed so dark matter and ordinary matter would fall together. Ordinary matter, being a plasma, dragged the radiation along with it, but as it collapsed, pressure would resist and cause re-expansion of ordinary matter (and the radiation along for the ride), producing oscillations. These oscillations could grow, but no faster than the speed of sound, probably about 2/3 the speed of light in a vacuum - but lots faster than the speed of light or other EM waves in the plasma. These oscillations produced slightly underdense bubbles with slightly overdense bubble walls.
The maximum size to which a bubble could grow was thus limited by the speed of sound and the age of the universe. When the plasma cooled to the point that neutral atoms could form, radiation was freed from its plasma shackles and made its way out into the larger universe, some of it eventually falling into one of our detectors of the Cosmic Microwave Background (CMB). Those bubbles, though, left their imprint on the CMB, 6 parts in 100,000 differences in the temperature.
They also left their imprint on the Universe. Those slight overdensities grew into the network of filaments and voids that now dominate the very large scale structure of the Universe. Cosmic expansion has blown up the bubbles or rings along with the rest of the Universe, and they are now some 490 million light years in maximum radius.
We also measure the recession velocities of the galaxies in the rings via their redshifts. The measurements of the distance to these standard rulers as a function of their redshift can be used to trace the expansion history of the Universe.
Gates, Evalyn (2009-01-24). Einstein's Telescope: The Hunt for Dark Matter and Dark Energy in the Universe (p. 232). Norton. Kindle Edition.