Question

I know what is the definition of interia but what "causes" it ??

Answer 1

Inertia is the property of a body. It is a property , just like mass.

What causes mass? This is an unanswerable question. This is because, mass is a property. Every particle has a mass, and this is a property, so is inertia.

Inertia is a property, that all particle with mass has.
Inertia is property by which a body continues to move or rest, if no external force is acted upon it.

The cause of inertia is mass. Every object with mass has inertia.

But. ,what causes inertia is unanswerable. If any object has a mass, it will have inertia.