> What else has approximately uniform local density Vacuum. Cool-phase neutral atomic gas in interstellar settings. Cool-phase neutral molecular gas ditto.

> What else has approximately uniform local density Vacuum. Cool-phase neutral atomic gas in interstellar settings. Cool-phase neutral molecular gas ditto. What breaks the uniformity of the latter two is mainly electromagnetic interactions. UV or X-rays will ionize them, and the freed electrons will cause secondary ionizations. This is the main pathway for heating neutral interstellar gases. Subsequent cooling is by photon emission. This drives dust-grain-forming chemistry; these grains are more dense than gas, and so have different gravitational observables as well as different emission/absorption characteristics. Very roughly, the dust grains can collide and stick to one another chemically, leading to further density non-uniformities. Cold dark matter doesn't feel UV or X-Rays or it wouldn't be dark, and if it is collisionless (as in the standard model of cosmology) then there is no dark chemistry that can locally densify dark matter. > What else has approximately uniform local density of distribution? Really, anything that approaches an ideal classical gas. "Local" is an important qualifier. A cubic centimetre of taken from near the middle of a small jar of water inside your household refrigerator or a cm^3 of gas taken from near the middle of a helium-filled balloon. The middle, because the density differs at the boundary of the material in the container. Likewise, the density changes sharply at the edges dark matter clouds, but is fairly uniform in large volumes far from the boundary. The overdensity in the jar's contents compared to the contents of the fridge overall and the underdensity of helium in a balloon in a room at sea level compared to the whole room are very roughly analogous to overensities and underdensities of dark matter at scales much larger than solar systems.