John Gill Commentary Exodus 10:21

"And Jehovah said unto Moses, Stretch out thy hand toward heaven, that there may be darkness over the land of Egypt, even darkness which may be felt." — Exodus 10:21 (ASV)

And the Lord said to Moses
About the eleventh day of the month Abib: stretch out your hand toward heaven ;
where the luminaries are, and from where light comes: that there may be darkness over the land of Egypt, even darkness which
may be felt ;
that is, what caused it, the gross vapours and thick fogs; for otherwise darkness itself, being a privation of light, cannot be felt: Onkelos paraphrases it, ``after that the darkness of the night is removed;'' so Jonathan; that it might appear to be different from that, and be much grosser.