What we see on small scales is actually sound. The photons behave as a gas just like air. Ordinary sound waves are just travelling compressions and rarefactions of the gas which we hear as sound as they strike our ear drum. The photons also carry sound waves as gravity tries to compress the gas and pressure resists it. The reason why we see it rather than hear it is that when we compress the gas it becomes hotter. We see the sound waves as hot and cold spots on the sky.
The result is a spectrum of sound waves that are useful in determining the origin, evolution and fate of objects in the universe.
