After some googling I'm somewhat amazed that there is no fundamental explanation for what causes inertia. models inertia as unruh radiation but it doesn't fit into existing theories yet (like QED), not to mention unruh radiation is theoretical as well.

quantized inertia would imply a very small but non-zero photon mass. It implies GR is wrong at larger scales but gets rid of the need for dark matter and energy. I can see why it is controversial.

Interesting post on photon mass from baez:

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