It's not the absence of light that makes the darkness so terrifying.

It's the unknowing.

There could be a monster with razor sharp teeth, blood already dripping from its claws. Or a serial killer, who has managed to evade the authorities for your entire adult life. Or, it could even be the Tooth Fairy come to collect on the I.O.U. from over two decades ago.

Or, could it be an apparition of your younger self staring back at you with nothing but disappointment?

Or, maybe, you're alone with your own thoughts, creating darkness within the light.