[villageidiot (OP)]

Regular matter is dispersed throughout the galaxy at very high density and low volume as compared to the vacuum of space. Massive objects such as stars, planets and nebula orbit around the centre of faster than they otherwise should if we only take into account the visible matter. It is theorised that dark matter holds the galaxy together with four to five times the gravitational force generated by the visible matter, therefore allowing the faster orbital speeds of the visible matter.

If our galaxy is awash with this abundant dark matter and it is gravitationally attractive why aren't those massive bodies in our solar system collecting the dark matter in their volume or orbit causing a larger dent in space-time. With time we should see the earth falling toward the sun, its visible mass plus accumulated dark matter forever increasing with time.

There are a couple of reasons why this hasn't happened.

Our solar system has only come into contact with very negligible quantities of dark matter.

Dark matter has an exceptionally high minimum volume compared with regular matter and cannot clump around astronomical bodies, causing a noticeable effect.

Dark matter doesn't gravitate toward regular matter but matter gravitated towards dark matter implying a one way relationship between the two.

Are any of these reasons correct? Or is this something that physicists don't yet know?

[Vern]

Dark matter is only speculation at present. Physicists attempt to fit observations into the physical rules that they accept. It may be that there's a missing rule, or it may be that the known rules are not correctly applied.

For example, we know that gravity affects the passage of time. The more gravity; the more slowly time passes. However, this known time-dilation effect is not considered in computations about the speed of stellar bodies in galaxies. If the equations were modified to include the time dilation due to gravity, the anomaly in galactic spin rate might go away.

There is another effect that is not considered. Galactic disks are in the range of 100,000 light years across and 20,000 light years thick. These monsters have been spewing light and nuclear remnants into space since they formed. All this stuff produces gravity. Yet; no one accounts for it.

[Soul Surfer]

Dark matter is gravitationally attracted to ordinarry matter that is the only way that its presence can be detected

The way that material that ONLY reacts to the gravitational force is very counter intuitive it is very similar to the difference between trying to move in space and trying to move on the earth's surface.  Just think of a dark matter particle ambling through a galaxy. It interacts with absolutely nothing except the gravitational field  ie it can go straight through ordinary stars and even neutron stars without noticing them it therefore passes through the gravitational field of a star a bit like a comet round the sun it accelerates as it approaches changes direction a bit and decelerates as it leaves.  If it was originally travelling very slowly and essentially falls straight towards the sun it loops tightly round the sun an makes its exit a few days later (at around the distance of the earths orbit) back along very close to the path it came in.  If it is going quite fast say significantly faster than the escape velocity at its closest approach to the sun it just follows a hyperbolic orbit and exits in a slightly different direction.

We are used to particles in gas where they are continually bouncing off each other and sharing out their energy but dark matter particles are probably about the size of a black hole of their mass and are so unlikely ever to interact that interactions between dark matter particles are almost non existent.

[Vern]

I would be inclined to dismiss the notion of dark matter altogether as not needed to explain the anomaly in the rotation speed of galaxies. But there was a recent observation that gives credence to dark matter. There are two galaxies that seem to have collided. The dark matter seems to have separated out from at least one of the galaxies.

http://news.bbc.co.uk/2/hi/science/nature/7587090.stm [Links inactive - To make links active and clickable, login or click here to register]

Astronomers detected the dark matter because it separated from the normal matter during the cosmic smash-up.

The research team are to publish their findings in the Astrophysical Journal.

They used the Hubble and Chandra space telescopes to study the object MACSJ0025.4-1222 - formed after an incredibly energetic collision between two large galaxy clusters.

[Soul Surfer]

That was not a collision between two galaxies it was a collision between two clusters of galaxies.  The evidence for dark matter is extremely compelling from galaxy rotations gravitational lensing and the cluster collision. 

[lightarrow]

[...]

There are a couple of reasons why this hasn't happened.

Our solar system has only come into contact with very negligible quantities of dark matter.

Dark matter has an exceptionally high minimum volume compared with regular matter and cannot clump around astronomical bodies, causing a noticeable effect.

What does "has an exceptionally high minimum volume compared with regular matter" mean? Maybe you want to say the correct thing, but you should say it better.

Anyway, the reason dark matter don't affect our solar system is that D.M. has a very low density, so in a volume of our solar system, the total amount of D.M. is negligible with respect to ordinary matter. In the volum of a galaxy is not negligible, instead. This because ordinary matter (as you say) is clumped in small regions and in the galaxy, on overall, there is more void (and so, D.M.) than ordinary matter.

[Soul Surfer]

That is a very good point light arrow.  most people just do not realise the relative size of our solar system compared with the gaps between the stars.  The distance to the farthest reaches of our solar system is measured in light hours or possibly one light day.  whereas the stars are all light years away.  this is a factor of around 1000 in linear distance while in volumes this represents 1000 cubed or on billion  so there is a billion times as much "empty space" as solar systems in our region of the galaxy.

[Vern]

That was not a collision between two galaxies it was a collision between two clusters of galaxies.  The evidence for dark matter is extremely compelling from galaxy rotations gravitational lensing and the cluster collision. 

Yes; the evidence is extremely compelling, but the concept of a mysterious unknown substance to explain the observations is not easy to accept. I'm not sure we have exhausted all possible explanations within the known substances. One thing I never see considered is the contribution to gravity by the photons and nucleons thrown out into the galactic vicinity. There's a few hundred thousand light years worth of the stuff that should have a noticeable effect.

[Soul Surfer]

The logical possibility of there being particle(s) that only interact by gravity is perfectly reasonable. 

My personal favourite for the majority of the mass is that there is some minimal sized black hole because as small primordial black holes get smaller by hawking radiation the event horizon may eventually get so small that nothing else other than a limiting black hole could ever get out it is also interesting to think that there may be rotating and non rotating versions of these particles

I expect that when things are worked out there may well be several  different types of particles may be involved including the supersymmetric ones.

[Vern]

You may be right, but there is a problem with the whole notion that a black hole can exist at all. It is difficult to model the formation of a black hole from an accretion disk, no matter how big a disk you start with. So that only leaves exploding stars, and none seem to exist that could create anything more than a neutron star.

Another thing I have never seen considered is the negative impact that gravity must itself have on gravitational force. Equations for that force contain a term for acceleration, which is time dependent. We know that gravity dilates time. Yet; we do not consider this. It should impact the ability of black holes to form. It should impact galactic rotation.

I expect that when things are worked out there may well be several  different types of particles may be involved including the supersymmetric ones.

[]

[Soul Surfer]

Stellar sized black holes do not form from an accretion disk. they form from a symmetrical collapse and pressure shock wave driven by the inertia of the matter that is collapsing.  Once the event horizon has been created it can then continue to grow using an accretion disk.

The very large black holes in the centre of galaxies probably formed relatively quietly very early in the life of our universe by straight forward collapse of very uniform and non turbulent material.  The bigger they are the "easier" it is to form a black hole provided they do not collapse into smaller objects and start radiating like stars.  Then the radiation tends to disperse the collapsing material.  Extremely small black holes could only be formed under the extreme conditions of the extremely early stages of the big bang probably under the influence of photon interactions or semi quantum processes.