A new approach is used to study the old question of missing mass in the Galactic disc. Invoking the law of large numbers, a simple average is formed of the many independent published values of the volume density of solar neighbourhood matter deduced from observational analyses of the gravitational force perpendicular to the Galactic plane. This average value is 0.15+/-0.01M_solar pc^-3, which should be compared with 0.10+/-0.01M_solar pc^-3 in known visible matter. The estimated 30 per cent dark matter significantly increases the Galactic z-force, thereby reducing the average half-period of vertical oscillation of the Solar system about the Galactic plane to 37+/-4Myr. A number of previously proposed Galactic mechanisms can both trigger and modulate the rate of gravitational perturbations of the Oort halo of comets. These mechanisms include encounters with massive interstellar clouds and the vertical Galactic tides. Predicted impacts of Oort halo comets on the Earth are broadly compatible with the terrestrial record of large impact craters. A specifically Galactic signature may appear in the 36+/-1Myr mean periodicity of cratering detected in Grieve's updated list of the ages of large impact craters.