system
Given the following post, respond as if you're commenting on Reddit:
human
What is the currently accepted theory (or theories, if several are competing) for the source of inertia?

I recently watched a Richard Feynman video in which he made the assertion that no one knows why inertia exists. Wikipedia confirms this view. When I did some research, the best I could find was a discussion on the vacuum field theory of inertia, but that was almost 16 years ago and I've never heard anything about it since.

What's our current best guess for where inertia comes from, and what experimental evidence (if any) supports each theory?

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*Update:* To be clear, what I'm asking is whether inertia a fundamental, irreducible property of matter in our universe (i.e., the "that's just the way it is" explanation), or is it an emergent property that hinges on other fundamental properties (i.e., the "inertia exists because of X, Y, and Z, and X, Y, and Z are fundamental properties of our universe" explanation)? And if it's the latter, what explanations do we have for why it might be an emergent property?

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Another reason I'm interested is because if inertia _isn't_ a fundamental property, then that suggests it might be theoretically possible to control, amplify, or negate it in some way, which I imagine would be an extremely interesting area of research.
gpt
The trouble with these kinds of questions is that you can say "no one knows why X exists" for absolutely everything.  You just dig down far enough.

E.g.  Why does gravity exist?  Why do electrons exist?  And so on.  Sure you can explain in terms of other things, but then you can just ask why those things exist, and repeat until there's no answer.

For inertia specifically, conservation of inertia is conservation of momentum plus conservation of energy.  So now we just need to explain those two things, and why things have any inertial mass at all.  The second is explained by the higgs particle - the usual colourful explanation being that the massive particle is like an attractive guy.  The more mass, the more attractive he is. And the higgs particles are obese women who crowd around the guy, making his progress hard.

So now you're asking why energy and momentum are conserved.  You can say those are symmetries of nature, but then what?  Why does nature have those symmetries?  Could nature have had different symmetries?