I Want To Answer But I’m Drawing A Blank

I recently went on a tour of a slate mine. Once we got deep into the mine, the guide asked everyone to turn their lights off so we could see how dark it was. He then lit a candle, which was all the light the miners would have had, but before doing so he told us a question a tourist had asked him a couple of weeks prior that he still didn’t have an answer to.

Tourist: “Can I take a photo of the darkness?”