That's "why" thing is new to me. I read before that there is no why, only how. "Why do particles exist? Why is something fundamental like it is?" Are these questions to be answered in ST framework?
Not a physicist, but questions of "how" and "why" are complicated by their ambiguity in English.
In common English, "how" means the mechanics of a system, and "why" generally means for what purpose someone has undertaken an action. Think of "Why did you build a garage?" (for what purpose or to what end?) vs. "How did you build a garage?" (what exact method did you use).
To complicate things, for action by non-agents, "why" can also mean "how". Children might ask "Why is the sky blue?" but what they mean (or the answer they get) is "How is the sky blue".
So when it comes to physics, we generally say there is no "why" (teleology, or mental purpose of an agent), only "how" (observable physical mechanisms). As far as physics is concerned, there is no God who set up the universe in a particular way, because he liked things a certain way, or he wanted a certain outcome.
However, the ambiguity of "why" can still be re-purposed, if you will, in physics. It's not referring to the will of an agency, but instead to lower-order phenomena that give rise to higher-order phenomenal. Why does the Earth sky have to be blue instead of, say, red? We're asking about why the mechanics of Releigh scattering play out one way verses another.
The whole point of general relativity is that there doesn't need to be a particle that does something, but that gravity is just an emergent property of space. So no, we don't know that a gravity particle has to exist.
The problem is that we have two formulations of physics. Basically, gravity and everything else. Theories such as QCD, QED, etc give rise to quantum mechanics, electricity and magnetism, etc. In these theories + and - charges for example feel an attraction to each other due to an exchange of "virtual" particles. General relativity is based on geometry as you say. Physicist would like to have one framework to describe everything--that's why it would be nice to find a graviton.
In common English, "how" means the mechanics of a system, and "why" generally means for what purpose someone has undertaken an action. Think of "Why did you build a garage?" (for what purpose or to what end?) vs. "How did you build a garage?" (what exact method did you use).
To complicate things, for action by non-agents, "why" can also mean "how". Children might ask "Why is the sky blue?" but what they mean (or the answer they get) is "How is the sky blue".
So when it comes to physics, we generally say there is no "why" (teleology, or mental purpose of an agent), only "how" (observable physical mechanisms). As far as physics is concerned, there is no God who set up the universe in a particular way, because he liked things a certain way, or he wanted a certain outcome.
However, the ambiguity of "why" can still be re-purposed, if you will, in physics. It's not referring to the will of an agency, but instead to lower-order phenomena that give rise to higher-order phenomenal. Why does the Earth sky have to be blue instead of, say, red? We're asking about why the mechanics of Releigh scattering play out one way verses another.