Exo 10:21 And the LORD said unto Moses, Stretch out thine hand toward heaven, that there may be darkness over the land of Egypt, even darkness which may be felt.
Exo 10:22 And Moses stretched forth his hand toward heaven; and there was a thick darkness in all the land of Egypt three days:
Exo 10:23 They saw not one another, neither rose any from his place for three days: but all the children of Israel had light in their dwellings.
Anyone that has been through a power outage can relate to the total darkness aspect of this. With an outage you get the flashlights and candles and lanterns out and wait. Then when the power is restored you sigh a sigh of relief and reset all the clocks. However, "Darkness that can be felt"??? You go to strike a match or flick your lighter and nothing. Not even a spark! No stars and moon at night for light either. Utter blackness all around, you cannot even see another person if they were 2 feet away. Your life is at a standstill. After a day of this I am sure you would start to feel like you were in a grave. No heat or cooking either because coal gives off a glow from the heat. Not only is this a physical fright but psychological and spiritual as well. The Egyptians' god of the moon was horus. The reason for three days and nights of darkness is because by law after three days of no signs of life, one was considered to be lawfully dead. Bodies were left for three days to be observed for signs of life before the embalming process was started. How did this effect Pharaoh? His brother and sister were considered the sun and moon.
Picture from here