It seems to be generally assumed that there is an extremely high pressure at the centre of a planet such as the Earth.  I don’t think this can be correct, surely there should be no pressure at all, a void.  Could someone please explain why my reasoning is wrong?Pressure is due to the weight of (for example) air or water pressing down, due to it’s mass and gravitational attraction.  Ascending, the weight of material above decreases, so decreasing the pressure.  Pressure rises (at first) when descending from the surface as the weight of the column of material above increases.But at the planetary core (or more correctly, the true centre of gravity) there can be no net gravitational force because the attractive forces of every particle in the planet (and indeed beyond the planet) cancel.  It is a zero-gravity place.  As one moves from the core, out of any void and towards the surface, the amount of material directly above becomes less and that below (through the core to the diametrically opposite surface) increases, so that a net gravitational pull towards the core develops, reaching a maximum at the surface.  There will be an intermediate point between the core and the surface where the weight of material above is at a maximum (due to net gravitational pull and depth of material above) and here the pressure will be at a maximum, decreasing both upwards and downwards.  At the exact centre, without any attractive force, there can be no pressure if the planet can be considered as an unrestrained system (and with fluid magma and tectonic movement this is surely the case, the planet is flexible).  It is very hot  in the centre, so particles in the vicinity will be thermally mobile and drawn to areas of gravitational pull, away from the very centre.  This effect will keep the core empty; to what extent I wouldn’t wish to speculate, but I should guess not a major part of the entire globe.  This low pressure and low density volume at the centre may be noticeable, and I wonder if this could explain some of the anomalous observations about planetary and solar cores.

If correct, this effect should be seen in many large agglomerations of fluid particles such as stars, or even a Galaxy.  (But NOT the entire Universe – it is expanding).  Perhaps there is more matter located towards the galactic periphery than in the centre, and this might help explain the anomalous galactic rotational speeds which have led to the postulation of dark matter.  Is the concept of dark matter actually needed if the galaxies have a different mass distribution to that usually assumed with gravitational pull a maximum somewhat away from the centre?However, I do wonder how the black holes that seem to inhabit the centre of many galaxies can remain ‘locked’ into position in the centre of an otherwise gravitationally neutral place.I feel that this whole concept is all too simple and somewhere I must have made a mistake, so I would really appreciate someone explaining why I am wrong.