Rabbi Moshe Ben-Chaim

Moses held out his arm toward the sky and thick darkness was upon all the land of Egypt for three days. No man saw his brother and no one rose from his place three days, but all Children of Israel had light in their dwellings (Exod. 10:22,23). 

Seeing one's fellow Egyptian and visiting him were both impossible during the Plague of Darkness, and this is significant enough to be recorded. That’s how God leads us to deep insights: He records only certain matters, and with precise wording.  Why did God cause these 2 affects?

Like all peoples, Egyptians sought reinforcement for their culture through camaraderie. One wishes to see his brother/countryman act as he does as this provides him security in their shared culture. As he sees others sharing his philosophy and his way of life, this emboldens his sense of validity in that culture. Common clothing and language further strengthen this conviction in one’s culture. But with this dark isolation, this identification and cultural validation was lost. Not seeing his brother, he would attempt to rise and find him, but this too the verse says he could not do because of the darkness. As Ibn Ezra said, “Where could he go?” Darkness forced everyone to stay in their place, and not endanger themselves.

This is why the last part of the verse says that the Jews had light in their dwellings in order to contrast the Jewish culture to the Egyptian culture. The Jews did not seek to bolster their beliefs through common culture.

Perhaps God’s objective was to first strip each Egyptian of his validation for Egyptian treatment of the Jews. This was achieved by 3 days of not seeing one’s brother. This opened the door to each Egyptian to reflect on himself as an individual. Isolation forces one to look at oneself and assess his life. Perhaps this contributed to the Egyptians’ remorse for Jewish slavery, driving them to give of their wealth when the Jews departed.