What is the origin of inertia? This is a question asked by many people. I believe even Einstein pondered on this and did put some of his thoughts in a paper, but nothing really conclusive. In Newtonian gravity, inertia appears to be just something that is inherent to mass. However, given relativity theory, this is no longer a satisfactory explanation. Inertia of an object increases with velocity according to relativity. (I assume that means inertia of a body also increases in a stronger gravitational field, though I have not seen that spelt out anywhere in my limited reading). So, if inertia increases with velocity or gravity, what is the source of the base inertia of a body? It appears that a body at rest and at ideal zero gravitation still has some base inertia. Where does that come from? I have come across some explanations like (a) Mach's principle and (b) expansion of Universe from Big Bang, as tentative theories of the origin of basic inertia. None seem to be convincing, esp. since there is no mathematical model to back up the explanations. Is there any comprehensive theory on the origin of inertia, or is it still a mystery of physics?