# Shell Theory 'paradox'; how am I thinking incorrectly?

I thought I understood the Shell Theorem and accept that it has great relevance to cosmology and the expanding universe. I follow the simple proof of this theorem (Newton's proof) but I'm left with some reservations. Firstly, the solution that there is no net gravitational force inside a spherical void in a medium seems to require the perfect cancelation of forces in all directions and distances; so inherently requiring that the medium is uniform over infinite distances.

My main concern though is that I can conceive of a situation which seems to leave a paradox that I don't know how to explain. Imagine that there are two spherical voids in the medium which just touch. At the intersection I would expect that there could be no net gravitational force on a small point located there since there is symmetry in all directions at this point and so however you might choose to estimate the net gravitational force it seems that there should be perfect cancellation at this location.

Now imagine that one of the two spherical voids is replaced by the medium. From the shell theorem it is clear that the effect of this added medium will be a gravitational force towards the center of the newly added mass (since Shell theorem states that in a uniform medium the effect of a spherical object is the same as a point mass at the centre of the object). With this mass then added back then I would expect there to be a gravitational force at the surface of the spherical void now 'filled in' towards the centre of that new object. However, if one simply considers the remaining void using the Shell theorem then it would be expected that there is no net gravitational force anywhere inside the remaining void.

So, how is my argument/construction with the two voids invalid? My suspicion is that the solution may relate to the impossibility of having a uniform medium of infinite extent, however I'm by no means able to resolve this myself. Can anyone help?