Great questions ! > How much [dark matter] [is] in the solar system ? A lot, because the solar system is a huge volume and dark-matter fills it fairly uniform

Great questions ! > How much [dark matter] [is] in the solar system ? A lot, because the solar system is a huge volume and dark-matter fills it fairly uniformly (there is a small overdensity inside the sun, and the planets will also cause small departures from essential uniformity). However, it's extremely sparse, so there isn't much in any small fraction of the solar system. Compare that with the planets: they are extremely dense, but do not fill more than the tiniest fraction of the whole volume of the solar system. However, even so, even Phobos and Deimos have much more mass than all the dark matter inside Mars's orbit (see a couple paragraphs below). ~ one third of a million proton-masses for every cubic metre ~ 6 * 10^-22 kg for every cubic metre (Earth's density is 524 kg/m^3) ~ 0.65 kg / earth volume http://cdms.berkeley.edu/Education/DMpages/FAQ/question36.html http://cdms.berkeley.edu/Education/DMpages/FAQ/question36.ht... Of course the total mass scales with volume. Inside Neptune's orbit there is about 10^17 kg of dark matter; that's about ten Phobos-masses, or about the mass of 253 Mathilde (a carbonaceous intermediate-belt asteroid). The volume of the galaxy is enormous, and with dark mater filling all of it roughly uniformly, the mass of all the visible matter is dwarfed -- there is an awful lot of space between star systems. > somehow observable It's not moving anywhere close to relativistically compared to the Earth's surface, and it doesn't feel electromagnetism. If we compare two other neutral particles, we have no practical ability to detect non-relativistic neutrinos (we can only spot a microscopic fraction of relativistic neutrinos from known sources) and have trouble spotting thermal neutrons (again, we generally need a known source that is "loud" with them, and additionally the collision momenta will still be larger than most collisions with solar system dark matter -- neutrons spit out of nuclear reactions are much faster than Earth's orbital motion through the extremely sparse dark matter the inner solar system sweeps through, and neutron beams used experimentally are generally a lot denser than ~ 3 neutron-masses per cubic centimetre).