In my experience, depression is emptiness.
Depression is a dark and empty expanse enveloping its host. While enveloped, the amplitude of your emotions decreases significantly. Happiness, sadness, and even anger become distant, as though you only experience their reverberated echo. This can go on for weeks, even months at a time. And when you are used to only feeling echoes, a real emotion sneaks up on you and it is deafening.
Sources for depression vary. Mine seems to be genetic. My suicidal thoughts started at the age of 9 and persisted until a series of unimportant events led me to get help. The pills took multiple months to kick in, but they didn't bring me happiness. Looking back, I was not ready for that yet. I needed something to stabilize me. To calm the intrusive thoughts in my brain so that I could be a functional human being again. One day I looked up at the sky and the clouds appeared beautiful. They brought me a subtle joy that I had previously lost any hope of feeling again.