Ordinary matter loses its kinetic energy through inelastic collisions, which allows it to gravitationally condense into star systems. Our best understanding of dark matter is that it is not able to cool in this way. Consequently, star systems are extremely overdense in ordinary matter but much less overdense in dark matter.

In particular, the local density of dark matter is expected to be about 0.4 GeV/cm^(3), i.e. less than a proton mass per cubic centimeter. The mass of dark matter that is relevant to the earth's orbit is then about 10^-17 times as massive as the sun. So dark matter affects the earth's orbit at the 10^-17 level, which is unfortunately not measurable.