[Roger Rowe]

Roger Rowe  asked the Naked Scientists:
   
Hi Chris & the Naked Scientists team

I've been listening to the show podcast for what seems like years now & find it very accessible & entertaining on my daily commute & long drives.

Here's a question:

If Dark Matter exists & has such a huge gravitational pull, how come that its not been attracted to all the regular matter and become part of the stars & planets like ordinary matter?  Surely we ought to be able to find the stuff here around us & not have to search the Milky Way and other galaxies for it.

Roger Rowe
Aylesbury, Bucks

What do you think?

[PhysBang]

In theory, it actually is around us. It just doesn't interact with ordinary matter except in a very limited way so it doesn't stay part of the structure of stars or planets. And it's hard to detect. Really hard.

[lightarrow]

If Dark Matter exists & has such a huge gravitational pull, how come that its not been attracted to all the regular matter and become part of the stars & planets like ordinary matter?  Surely we ought to be able to find the stuff here around us & not have to search the Milky Way and other galaxies for it.

Because it has very low density, so on little volumes (our solar system) the total amount of DM is too low to affect the planets' motion or other.
On larger structures (a galaxy) the total amountof DM is significantly larger than that of visible matter (stars and planet systems are very far away one from the other).

[Soul Surfer]

As others have said dark matter only interacts by gravity, so, as a dark matter particle approaches a star from a great distance even if it is going very slowly it accelerates towards the star like a comet and by the time it gets to the surface of the star it is going at a great speed it then passes quickly through the star not bumping into anything on the way in a parabolic or hyperbolic orbit  (except as it is passing through the star where the gravitational field is varying) and vanishes back out into space.  The overall effect is a slight change in the overall density of dark matter near strongly gravitating objects like stars planets, galaxies and clusters of galaxies. This causes the gravitational lensing that allows us to see the most distant objects in the universe.

This is one of the several less well understood things about gravity.  Nothing it in the universe would ever condense into objects if they can't lose energy by interacting in other ways to lose energy.