It sounds like dark matter makes up one quarter of the universe's mass-energy. And it's density is fairly uniform. But could it be simultaneously both uniform and moving - like currents in a lake. Does it have a standing wave? A propagating wave? A rotation around some universal central point? Local rotations around massive objects like low pressure systems on a weather map?


Yes and no. In general, Dark Matter tends to exist in the same places as galaxies. It isn't really distributed throughout all of space uniformly. It has voids and filaments just like normal matter does. In fact, on the whole, normal matter is a pretty good tracer of where Dark Matter is. In some cases Dark Matter can separate from normal matter, and it's believed by some that Dark Matter can clump in areas where normal matter doesn't, but more or less, on a cosmological scale, it is distributed and acts like regular matter. On short timescales, Dark Matter most just sits there. There isn't mass motion or propagating waves. On long timescales and in very large views, you can see actual motion and interesting structures (e.g., Baryonic Acoustic Oscillations - technically we observe these in normal matter, but Dark Matter would be affected in the same way).

To give you an idea of this, check out the Millennium Simulation (that's a YouTube video which moves around and views the results of the simulation). This was a set of simulations of the universe as a whole, in an attempt to test out current theories about our universe and see if those theories could produce a universe that looks like ours. Suffice to say, they were pretty successful. An image of the results of the simulation is shown below. This represents the universe at a very very large scale and you can see all the dark matter and how it is distributed.

enter image description here

You might also want to check out the Aquarius Simulation which was very similar in nature to the Millennium Simulation. However, unlike the video for the Millennium Simulation, this video shows the evolution of the universe. This really gives you an idea of the flow and motion of dark matter over the entire history of the Universe.

  • $\begingroup$ Good answer, I was just about to type out something similar when I noticed your answer! Just for some reference it might help to quote an estimate of the density of dark matter for scale. Its not easy to find a consistant value but I did notice this discussion on the dark matter density in the Solar System physics.stackexchange.com/questions/102605/… $\endgroup$
    – Dean
    Jan 26 '17 at 16:32
  • $\begingroup$ @Dean Feel free to post your answer as well. I'm sure there's plenty more to be said that I didn't touch upon. $\endgroup$
    – zephyr
    Jan 26 '17 at 16:33
  • 3
    $\begingroup$ A detail: It's the other way around actually, normal matter tends to clump around DM. As DM dominates the matter content of the universe, baryons must follow DM. $\endgroup$ Jan 26 '17 at 20:47
  • $\begingroup$ Great answer, but note that the yellow stuff isn't really normal matter, or baryons, although baryons do clump there. The Millennium Simulation doesn't contain baryons, it's a "dark matter-only" simulation. The color simply denotes density, with {black, red, yellow} = {low, intermediate, high} density. Also, I agree with @AtmosphericPrisonEscape that the sentence "dark matter tends to clump around galaxies" sounds like DM follows the baryons, whereas it's more the other way round, since DM dominates over baryons by a factor of >5 in mass. $\endgroup$
    – pela
    Jan 26 '17 at 21:05
  • $\begingroup$ What do you mean by "On short timescales, dark matter most just sits there"? $\endgroup$
    – ProfRob
    Jan 28 '17 at 7:52

Hypothesised, weakly interacting, cold dark matter is affected by gravity, but behaves differently to normal matter, in terms of its spatial distribution and kinematics, because it is dissipationless. That is, it cannot lose kinetic energy through interactions and radiate heat away.

The distribution and motion of dark matter can be looked at on a number of scales. On the scale of galaxy clusters and superclusters, dark matter is arranged into voids and filaments, as shown by Zephyr's answer. It is decidedly non-uniform.

On the scale of a galaxy, our Milky way galaxy for instance, dark matter is thought to be distributed in a smoothly spherical, centrally concentrated way, and extends well beyond where the normal matter is found. The dark matter is moving; it orbits in the overall Galactic gravitational potential like everything else, but the orbits will be much more radial, rather than the circular orbits followed by stars in the disk of our Galaxy.

What this means for dark matter on solar system scales, is that we expect there is a dark matter "wind", because the Sun is orbiting at around 200 km/s with respect to the Galaxy, and so the dark matter will have this large number superimposed on its relative velocity with respect to the solar system. The wind at the Earth will also get stronger and weaker by 30 km/s as the Earth travels in its orbit.

The spatial distribution of dark matter on solar system scales should be quite uniform, and be less than one hundredth the density of normal matter in interplanetary space. There should however be a small (of order 1% density contrast) gravitational focusing effect due to the Sun as the "wind" passes by it.

  • $\begingroup$ Using the colours of the video, the Milky Way is located at one of the yellow spheres of d.m. It sounds like the spiral arms of normal matter are rotating but the underlying d.m. sphere is not. At the galaxy scale, the d.m. is moving per the structure of the d.m. voids and filaments. The spiral arms are like the water spinning around the bathtub drain while the d.m. tub itself is not. $\endgroup$
    – scorpdaddy
    Jan 28 '17 at 23:05

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