system
Given the following post, respond as if you're commenting on Reddit:
human
Have the amount of Dark Matter changed with the age of the universe?

As in, if we look back in time at galaxies far away, do they seem to have as much dark matter as closer galaxies do? Shouldn't dark matter be more "clumped up" in the early days of the universe, since it hadn't expanded as much?
gpt
All you said is correct. Basically, the number of dark matter particles in a comoving region of the Universe is conserved (in the simplest models and after derelativistisation). However the volume of the region is increasing due to cosmological expansion. Therefore the number density (which is proportional to the mass/energy density) decreases as a^-3 , where a(t) is the scale factor describing the "size" of the Universe at time t.

So dark matter decreases in density on average.