Types of Stuff


Electromagnetic radiation, Wavelength stretches with expansion of space. Dilutes as fourth power of the expansion.

Matter. Dilutes as third power of the expansion.
Ordinary matter - protons, neutrons, electrons.
Dark Matter - stuff that feels gravity and clumps together, but does not interact with ordinary matter, or itself.

Dark Energy - another name for an intrinsic energy density of space, aka Vacuum Energy aka Lambda. Does not dilute at all with the expansion.