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Recently I read an article that there is dark matter around the sun but if it is so, than why can we see it clearly.

If it is called matter than it shall show some hindrance in radiation we receive and the solar flares must bounce back against them though they are coming straight to us.

To stay close to sun it has to be rotating at a very very high speed, so does it?

Does it pose some kind of gravitational effect on the rest of the system?

Is it constant or does it increase or decrease in size?

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It's unlikely there is an especially high concentration of dark matter in the Solar System.

The principle attribute of dark matter is that it's weakly interacting. It's not just weakly interacting with normal matter, it also interacts only weakly with itself. Trying to get matter to collapse into a ball like the Sun is harder than you might think. The matter falling into the gravitational well has a lot of kinetic energy, and you can only make it aggregate in the centre if you can dissipate that energy somehow. If you don't dissipate the kinetic energy the infalling matter just whizzes straight through and out the other side.

Ordinary matter can dissipate energy by radiating photons i.e. it gets hot and loses energy by shedding heat. Dark matter can't do this, or rather it can do it only extremely slowly because it has no interaction with electromagnetic fields and can't radiate photons. This means it can't easily be concentrated in the gravitational field of a star, and consequently the dark matter density near the Sun isn't likely to be much higher than the average density in this part of the universe.

You do get regions where the dark matter density is higher than average, for example the bullet cluster. The question is whether the galaxy cluster attracted the dark matter or whether the cluster formed because the dark matter density was already high in that region (possibly as a result of fluctuations left behind by inflation).

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  • $\begingroup$ John, I was just about to formulate a similar question, when I came across this. Why can't the Sun capture dark matter, given that the escape velocity at the solar surface is likely greater than the orbital speed of dark matter round the Galaxy? $\endgroup$
    – ProfRob
    Commented Mar 16, 2015 at 23:12
  • $\begingroup$ @RobJeffries: if the dark matter starts at an (effectively) infinite distance from the Sun then unless it can shed kinetic energy it will execute a parabolic/hyperbolic orbit around the Sun and finish an infinite distance away from the Sun. It's the shedding kinetic energy that's difficult to achieve for weakly interacting matter. $\endgroup$
    – John Rennie
    Commented Mar 17, 2015 at 6:14
  • $\begingroup$ John -of course. Some inelastic interaction is required. $\endgroup$
    – ProfRob
    Commented Mar 17, 2015 at 7:24
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Dark matter doesn't mean that it occludes light. It means that it doesn't interact with light. Therefore, it also can not block light. Light just goes through it as if nothing was there. Dark matter only interacts gravitationally. That's how we know it must be there.

It does not necessarily have to rotate at high speed, but if it doesn't, it will just cumulate in the center of the Sun. Since that could lead to a gravitational collapse of the Sun, it must be that either that dark matter does posses rotational speed, or it is not present in sufficiently high densities to have a very notable effect on the Sun. It's probably a combination of both.

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  • $\begingroup$ than why it is dark matter...it should be transparent matter ? $\endgroup$
    – Pankaj Sejwal
    Commented Aug 16, 2012 at 13:59
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    $\begingroup$ I guess transparent matter would work, it just doesn't sound that ominous. ;) $\endgroup$
    – Raskolnikov
    Commented Aug 16, 2012 at 14:35
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    $\begingroup$ "Dark matter only interacts gravitationally." Dark matter is only known to interact gravitationally. The favored candidates also interact via the weak interaction. Of course, that theory is favored because it is the one that we can check on. Looking for our keys under the streetlamp and all that. $\endgroup$
    – dmckee --- ex-moderator kitten
    Commented Aug 16, 2012 at 14:52
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    $\begingroup$ Fair enough, I think every statement about dark matter should really be preceded by "we think that..." or "it is hypothesized that...". $\endgroup$
    – Raskolnikov
    Commented Aug 16, 2012 at 16:17

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