The only fully honest answer to this question: it is not understood. A good chunk of physicists (not all) would agree that the current understanding is unsatis

The only fully honest answer to this question: it is not understood. A good chunk of physicists (not all) would agree that the current understanding is unsatisfactory, but it's unclear whether now is the "right time" to research that question. It is unclear whether we have the experimental tools to probe the consequences of the different models, and whether different approaches produce different predictions in some testable scenario. Of course, there is an active field of research probing this and related questions. Different physicists in there have different ideas, depending on how they interpret quantum mechanics. (See other responses in the thread for a sampling of those). But there is no satisfactory agreed-upon textbook answer. Personally, I find the some aspects of the decoherence perspective appealing.