Gravity

Gravity is caused by rotation of large objects.  Mass alone does cause gravity, but an object of large mass and surface area will cause gravity based on the speed of its rotation generating force.  This gravity will affect other bodies in space, causing moons and asteroids to rotate and orbit, which create their own less powerful gravity due to the smaller size of these objects.  Multiple small objects could form a loose surface area with some gravity generated from their speed and rotation.  

The Earth's gravity is not caused by a "spinning molten core," but by the rotation of the entire mass of earth causing gravitational force based on the size of its surface area.

Absense of rotation equals zero gravity.  If the Earth stopped rotating, we would all start floating towards space.

Well, what about light?

In reality, light has mass and can be affected by gravity. BUT, light does not rotate!

Why?

Because light does not rotate, it does no exert gravity. But since it is matter, it is still affected by the gravity of other things. Hence why light can bend into a black hole, but light waves do not cluster together in their travel between galaxies. (as they should if they exerted gravitational force)