A Soulful Journey

The Plague The deepest darkness is when a person doesn't see his neighbor and doesn't participate in the sorrow of another person: "A person didn't see his brother". The consequence is that, when one doesn't feel his fellow's sorrow, the feelings become dull and are silenced. "No one got up from beneath it". He has no way of recovering. Hiddushei HaRim to Ex. 10:23, cited by Itturei Torah

Unlike all the other physical phenomena that engage us with the world - touching, feeling, seeing, or smellling - darkness is merely the absence of something, light. In the ninth of the ten plagues Moses brought down a new kind of Darkness that could be felt.  It was akin to a knocking on the door which we do not see but nonetheless feel a presence.  It was a Darkness of the Spirit.