Language and manners

[Rein's POV: ]

Darkness is absence of light.

When all colors are absent, when light leaves a space completely, when everything that remains is nothing, darkness is born at that moment, in that space.

Just like how Mana is present everywhere, darkness might also be present everywhere in a dimension where light could or does not reach.

'What we call darkness is absence of light.'

What we call shadow on the other hand, is the obstruction of light by a physical object.

'Light that is present everywhere.'

And the light that gives way to everything that exists and makes everything not only visible but also available to an individual, exists for everything. 

Shadows are created because of the obstruction of an object but that's just the basic understanding of what we call a shadow.