what do christians (and people in general) mean by "feeling empty inside"?

[Angrboda]

I think the idea of a "God shaped hole" or that feeling of emptiness inside likely points to a phenomenologically real thing, that there are specific subjective states which feel like that, though looking at responses such as GC's, I suspect it's used most often simply as an apologetic "stub" by people who may not ever have even had the experience. I suspect if you asked 100 Christians to describe the experience, you'd get all sorts of descriptions of anything from the mundane feelings of sadness to the sublime heights of existential angst, such as those pierced by Keirkegaard and Christian mystics. Various religious thinking triggers brain functions associated with interpreting the existence of others, other minds, others' intentions, and the self. (e.g. Jesse Bering's work on the belief instinct, and the observation that fMRI scans show that when a person is thinking about what God would want, they are activating the centers that activate when thinking about what they would want.) So I suspect it's a real phenomenon, seemingly related in a way to Sartre's look of 'the Other', but I doubt the Christian interpretation of its metaphysics and psychological meaning have any validity. But then, that's typical of religion, and especially modern Christianity, to take a powerful emotional experience and graft it onto an existing metaphysic, thus satisfying the "sounds reasonable" center of people's brains. (As a side note, Buddhism is guilty of much the same thing in its approach. It takes common emotional experiences and welds a conjectural psychological and metaphysical explanation onto those experiences, to encourage adoption of the religious practices [from meditation to specific ethics and politics].)