Replace this code:
particle.position.x = Math.random() * 2 - 1;
particle.position.y = Math.random() * 2 - 1;
particle.position.z = Math.random() * 2 - 1;
particle.position.normalize();
particle.position.multiplyScalar( Math.random() * 10 + 450 );
With this:
particle.position.x = Math.random() * 500 - 250;
particle.position.y = Math.random() * 500 - 250;
particle.position.z = Math.random() * 500 - 250;
This code places the points at the edges of the box:
particle.position.x = Math.random() * 2 - 1;
particle.position.y = Math.random() * 2 - 1;
particle.position.z = Math.random() * 2 - 1;
particle.position.normalize();
particle.position.multiplyScalar( 500 );
particle.position.clamp(
new THREE.Vector3( -250, -250, -250 ),
new THREE.Vector3( 250, 250, 250 )
);