Ask someone to describe darkness and you will receive different definitions. Rabbeinu Bachya in interpreting this plague gives three examples. There is tangible darkness, thick darkness, and darkness that will materialize. Each one of these is experienced differently.
Moses walked up Mount Sinai in darkness, a sense of fog. The darkness that the Egyptians experiences was one that would continually deepen. Bachya explains that this darkness began when it was light already, for if it was the other way around, it would have appeared as a natural phenomenon. The midrash notes that this darkness could be touched.
Each morning after we say the barchu, we thank God, yotzer or uvoreh chosehch, the creator of light and dark.
We can only understand light if we know darkness. We know at the end of our day, darkness will fall upon us, but we must also recognize that light comes with the dawn. It is no mistake that the Rabbis allow us to say the shema when you have the ability to see a person in front of you, for when that is possible, we have the ability to once again make our declaration of faith that today can be a good day.