This plague consisted of a thick, heavy darkness that fell across the land of Egypt for three days (Exodus 10:22). This must have been an eerie blackness with no sliver of light, similar to the total darkness a person experiences deep within a cave when all of the lights are turned out. It was so frightening that the Egyptians did not venture outside their houses (Exodus 10:23).
Once again, the Lord proved His superiority over the Egyptian gods. The Egyptians worshiped the sun god through the sun got Ra (see note on Genesis 12:15). But God was master over the sun and all other forces or nature.
EXODUS 10:21- 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.