"Darkness" is the idea that there can be 'diminished light', or indeed 'no light'.
"Light" is the light of awareness. God is light. Your True Self is light.
So in order to seemingly block or shut out the light, which is what takes place in 'the separation'/original sin, there has to be some kind of belief that darkness can actually have some kind of power over light - that it can cause light to go away.
This is actually a completely insane belief because it makes absolutely no sense. We know that light dispels darkness. Light shines away the darkness. When there is enough light there aren't even any shadows. Where does the darkness of night go when the sun comes up? It is shone away and does not exist.
But what the ego is saying is, that darkness actually has a 'power' or some kind of way in which it can actually defend itself against the light, stop light from shining, make light go away, cover up light, and in fact actually 'drain' light to the point of destroying light, so that there is no light left. This is the ego's insane idea which comes from the idea of separation from God.
All of the ego's entire thought system, all of its 'world', all of the separation and everything in it, arises from this idea of being able to somehow block God's unlimited, unblockable light.
So what we end up with then, is this strong belief that somehow, darkness is CAPABLE of concealing the light, preventing the light, even removing the light. And this is very frightening. But it also now means that, the darkness is HIDING SOMETHING.
Believing the darkness has the power to hide, and that it IS hiding, means there is something in the darkness 'with you' that you cannot see, and that is terrifying.
This literally then, is, "fear of the dark". i.e. fear of what is IN the dark. But as Jesus says, even the belief that darkness can hide at all is enough to induce fear. Especially since you don't know IF there is maybe something in the dark that you can't see, something outside your awareness or control, that could strike at any moment.
This is what the ego loves. This idea that it has hidden the light of God, hidden who you are, hidden your home, hidden your mind, in a great thick solid-seeming darkness in which there is no light and nothing can be seen for what it is.
Such is insanity, because in truth, light always dispels darkness. The light includes everything, the darkness includes nothing. God's light cannot be removed or hidden, in reality, or in truth, but only in an insane nightmare dream of darkness and death.
You are still the light.