כא) וַיֹּאמֶר יְהוָה אֶל מֹשֶׁה נְטֵה יָדְךָ עַל הַשָּׁמַיִם וִיהִי חֹשֶׁךְ עַל אֶרֶץ מִצְרָיִם וְיָמֵשׁ חֹשֶׁךְ.
כב) וַיֵּט מֹשֶׁה אֶת יָדוֹ עַל הַשָּׁמָיִם וַיְהִי חֹשֶׁךְ אֲפֵלָה בְּכָל אֶרֶץ מִצְרַיִם שְׁלֹשֶׁת יָמִים.
כג) לֹא רָאוּ אִישׁ אֶת אָחִיו וְלֹא קָמוּ אִישׁ מִתַּחְתָּיו שְׁלֹשֶׁת יָמִים וּלְכָל בְּנֵי יִשְׂרָאֵל הָיָה אוֹר בְּמוֹשְׁבֹתָם.
(21) And the LORD 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.’ (22) And Moses stretched forth his hand toward heaven; and there was a thick darkness in all the land of Egypt three days; (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.
Sources:
1) Ramban:
לֹא רָאוּ אִישׁ אֶת אָחִיו וְלֹא קָמוּ אִישׁ מִתַּחְתָּיוNo man could see his brother nor could anyone rise from his place.
Ramban asks; why could the Egyptians not move just because it was dark?
This darkness was not just that there was no sunlight. It was more than a nighttime darkness. It was “thick darkness” meaning that a very thick midst came down from the sky. This is why Hashem said to Moshe “Stretch forth your hand to the heavens” - Moshe had to bring down this darkness.2) Seforno:
וְיָמֵשׁ חֹשֶׁךְ.Seforno comments on these words. This will remove the normal darkness called “night”. The reason this was necessary was that the night consists of air ready and capable to absorb light in the morning. The darkness that would occur now was something unable to interact with light at all. The reason for this inability to interact with light was the density of the texture of this darkness. As a result of this totally different kind of darkness even a lit up flare would not be able to make a “dent” in the darkness3) Or Hachayim - Rabbi Chayim Ben Attar:
וּלְכָל
וּלְכָל
All of Israelites had light
He comments on this word. No matter what a Jew had light. Whenever a Jew went to the house of an Egyptian he still had light even within the dwellings of the Egyptian.