When you create a strong back scattering effect, it's important to pay attention to the geometry that sits between the shaded object's front and back surfaces, since it will block scattered light and be visible in the scattering effect.

In cases where you actually want to block light — to create a silhouette inside of a shaded object, for example — you can apply the exterior object's material to the interior object. This ensures that both objects are included in the lightmap computations, and participate in the scattering effect. The interior object will block the light and become more visible as you increase the back-scattering depth.

The scene below contains a brightly backlit cube with a cone inside of it.

When both objects are shaded with the same Fast Subsurface Scattering material, the inner object blocks light scattering, creating a silhouette inside of the outer object.