After some googling I'm somewhat amazed that there is no fundamental explanation for what causes inertia. https://en.wikipedia.org/wiki/Quantized_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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