# Is dark matter moving faster than light? [closed]

My way of thinking is as follows:

Consider a balloon. Let this be blown into a nearly elliptical sphere. Now, the temperature inside the balloon is increased by some means; this leads to the expansion of the gas and eventually the balloon. So if we consider the shock caused during the Big Bang as the cause similar to the temperature rise in the balloon, and if we also consider dark matter as the gas particles which is moving with divergence, and the galaxies can be assumed to be at the opposite sides of the balloon (though the sphere doesnt have any opposites), in this balloon case the expansion will be proportional to the distance; this model fits exactly into the universe model. So is dark matter moving?

## closed as unclear what you're asking by ACuriousMind♦, John Rennie, Kyle Kanos, Constandinos Damalas, David Z♦Mar 29 '15 at 15:56

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• Hi you ask is dark matter moving faster than light ...short answer, no, nobody has demonstrated this to my knowledge. Regards – user74893 Mar 29 '15 at 11:49
• please read upe on what is dark matter en.wikipedia.org/wiki/Dark_matter and also contrast it with dark energy: en.wikipedia.org/wiki/Dark_energy – anna v Mar 29 '15 at 11:49

This is yet another instance of taking the ubiquitous balloon analogy too far.

See, while it's a wonderful way to express the expansion of the universe, there are some misconceptions that arise from it:

• We live in a universe of finite size (we don't know, but we think not) and non-zero curvature (according to WMAP, we don't, or at least we think we don't)
• There are higher dimensions in which our 3+1-dimensional "universe" is embedded (there may very well be, but they aren't necessary for an expanding universe)
• There is something inside the balloon that is causing the expansion.

The latter pops up (in my experience) less often that the others, but it seems that you have hit upon it. It seems that you're treating dark energy as the "something" expanding the balloon.

First off, dark energy is not the cause for the expansion of the universe. When I was younger, I used to think this, but it's absolutely false. The universe can go on its merry way, expanding, without dark energy. But to accelerate that expansion, we do need something else. That something else is dark energy.

Ah, but I need to backtrack. I discussed dark energy, not dark matter. Well, that's because I think you mixed the two up, as anna v so helpfully pointed out. Dark energy causes the acceleration of the expansion of the universe; dark matter causes weird galactic rotation curves. We know that dark matter is in the Milky Way - and in loads of other analogies. So even if you didn't make a typo, the premise is incorrect.

Is dark matter moving? Well, relative to what? There's no absolute reference frame, so we have to clarify what reference frame we're discussing.

If we're referencing

• another galaxy, then the answer is yes, because the Milky Way is moving with respect to the reference frame of that galaxy.
• the stars in the Milky Way, then the answer is also yes.

In fact, if I say that the reference frame is in a galaxy which is receding from us faster than light, then dark matter is indeed moving faster than light. Sort of. But so are we.

And yes, dark matter moves (linked in a comment by lemon).

• Do we really know if and how is the dark matter moving(relative to stars, galaxies and other arbitrary things)? – user74200 Mar 29 '15 at 16:00
• @user74200 I can't list any empirical evidence off the top of my head, but there's no explanation for why dark matter should influence the motion of other objects, while its motion would not be influenced. – HDE 226868 Mar 29 '15 at 16:03
• @HDE_226868 yeah, sure, according to laws of gravity, it should be moving, but we don't have any experimental evidence of its movement, do we? It can move in very complicated way or very simple way, it could be at rest relative to some stars and move at great speeds relative to other, doesn't it? – user74200 Mar 29 '15 at 16:18
• @user74200 The distribution of gravitating matter necessarily tells you something about it's dynamics, whether it is dark or not. So the dark matter near the Sun orbits the Galaxy at roughly 250 km/s. If it didn't it would not be near the Sun. – Rob Jeffries Mar 29 '15 at 23:20
• @user74200 See this. – HDE 226868 Apr 1 '15 at 20:26