Why does existence exist?
Answering the question by reference to any particular ‘thing’ that exists (a ‘force’, ‘singularity’, ‘multi-verse’, ‘string’, etc.) is to completely not pay attention to the question. The answer cannot be in terms of any merely-existing thing, but must be in reference to some ‘more-than-existing’ kind of ‘more-than-thing’. Phrases like ‘ground for existence’ or ‘foundation of the universe’ are appropriate attempts here.
The fact that these are metaphors shouldn’t surprise us. (After all, even the most ‘technical’ and ‘precise’ terminology is metaphor at bottom anyway…) It’s quite obvious that the universe doesn’t have a ‘foundation’ like a house; and it would seem obvious that ‘existence’ isn’t on top of some ‘ground’ in the same way that we might be at times. But it remains that answering a question about why existence exists demands reaching for a category larger (or more ‘foundational’) than existence itself. If asked ‘what is supporting that house’, could we really be satisfied with an answer that was in terms of house-ness?