Question

What does cause gravity to exist?
What does cause Inertia to exist?
I doubt:
Inertia of moon cause it to move by straight if the gravity between earth and moon don't attract each other.
Where do gravity and inertia come from?
Everything has its own reason.​

Answer 1

Explanation:

Earth's gravity comes from all its mass. All its mass makes a combined gravitational pull on all the mass in your body. That's what gives you weight. And if you were on a planet with less mass than Earth, you would weigh less than you do here.

The cause of inertia is the resistance offered by the body/object to change in its state of rest or motion. Note: Newton's first law of motion states that a body at rest or in motion will remain in that state until and unless it is compelled by an external force.