Dark matter

MNyberg1020

If dark matter really does exist everywhere, and is constantly passing through our bodies, why doesnt this extremely dense matter affect regular matter? How are the atoms of our body not pulled by dark matter's gradational field? Also, if dark matter is thought to exist because stars in galaxies are moving too fast to be held in orbit by regular matter alone, why doesn't our solar system demonstrate those same properties if dark matter is everywhere? Why do the velocities of our planets drop the farther they are from the sun? Shouldn't our planets be affected by dark matter too?

Nabeshin

I've done the crude analysis in the past somewhere on this forum, but I don't feel like digging up the post... The point is, if you do the calculation, you find out that the total mass of dark matter within our solar system should be ~10^19kg, roughly uniformly distributed between the sun and the outer edges of the Kuiper belt. Now, 10^19kg is around the mass of a medium sized object in the asteroid belt. So, to begin with, the gravitational perturbation from such an object is tiny, but then considering that it is smeared out over dozens of AU makes the effect much more difficult to detect.

Janus

If you were to take the mass of the Sun, planets, etc of the solar system and divide it into the volume of the solar system, the average density you got would be much much greater than if you were to take the mass of all the star systems in a large local section of the galaxy and divide it into the volume of that section. In other words, the Solar system is a dense pocket compared to the local region of the galaxy.

Now consider that the vast majority of visible mass of the galaxy is constrained to a disk and dark matter is spread out in a spherical volume enclosing that disk. So even though there is ~5 times as much dark matter than visible matter in the galaxy, it is not dense, but rather spread out over a much larger volume and very thinly. So as Nabeshin has already said, you would expect to find very little dark matter within the Solar system.