Midrash Tanchuma, Bo 1

‎וַיֹּאמֶר ה' אֶל מֹשֶׁה נְטֵה יָדְךָ עַל הַשָּׁמַיִם וִיהִי חֹשֶׁךְ עַל אֶרֶץ מִצְרָיִם וְיָמֵשׁ חֹשֶׁךְ. זֶה שֶׁאָמַר הַכָּתוּב: שָׁלַח חֹשֶׁךְ וַיַּחֲשִׁךְ וְלֹא מָרוּ אֶת דְּבָרוֹ (תהלים קה, כח). חֹשֶׁךְ שֶׁשָּׁלַח הַקָּדוֹשׁ בָּרוּךְ הוּא עַל הַמִּצְרִים, קָשֶׁה הָיָה. לָמָּה? עַל שֶׁלֹּא קִבְּלוּ מָרוּת דְּבָרוֹ שֶׁל הַקָּדוֹשׁ בָּרוּךְ הוּא. אָמַר הַקָּדוֹשׁ בָּרוּךְ הוּא לַמַּלְאָכִים: הַמִּצְרִים רְאוּיִין לִלְקוֹת בַּחֹשֶׁךְ. מִיָּד הִסְכִּימוּ כֻּלָּן כְּאַחַת וְלֹא הִמְרוּ אֶת דְּבָרוֹ. שָׁלַח חֹשֶׁךְ וַיַּחֲשִׁךְ, אוֹתוֹ חֹשֶׁךְ נָתַן בּוֹ מַמָּשׁ. מָשָׁל לְמַה הַדָּבָר דּוֹמֶה? לְמֶלֶךְ שֶׁסָּרַח עָלָיו עַבְדּוֹ. אָמַר לְאֶחָד: לֵךְ וְהַכֵּהוּ חֲמִשִּׁים מַגְלְבִין. וְהָלַךְ וְהִכָּהוּ מֵאָה, וְהוֹסִיף לוֹ מִשֶּׁלּוֹ. כָּךְ הַקָּדוֹשׁ בָּרוּךְ הוּא יִתְבָּרַךְ שְׁמוֹ, שָׁלַח חֹשֶׁךְ עַל הַמִּצְרִים. וְנִתּוֹסַף הַחֹשֶׁךְ מִשֶּׁלּוֹ. הֱוֵי, שָׁלַח חֹשֶׁךְ וַיַּחֲשִׁךְ. And the Lord said unto Moses: “stretch out thy hand toward the heaven, that there may be darkness over the land of Egypt, even darkness which may be felt” (Exod. 10:21). Scripture states (elsewhere in reference to this verse): He sent darkness, and it was dark; and they rebelled not against His word (Ps. 105:28). The darkness that the Holy One, blessed be He, spread over Egypt was exceedingly thick. Why? Because they would not submit to the authority of the word of the Holy One, blessed be He¹Word-play on rebelled not (maru), connecting marut (“rebel”) from marut (“authority”). See Exodus Rabbah 4:1. The Holy One, blessed be He, told Egypt’s guardian angels: They deserve to be smitten with darkness, and they all agreed at once, for they rebelled not against His word (ibid.). He sent darkness, and it was dark (ibid. 105:28). This implies that the darkness had a substance of its own. To what may this be compared? It may be compared to a king whose slave has rebelled against him. He told one of his aides: “Go give him fifty lashes.” When that person whipped the slave, however, he administered a hundred lashes, adding fifty of his own accord. Similarly, when the Holy One, blessed be He, may His name be blessed, sent the darkness upon Egypt, the darkness added something of its own. Hence, He sent darkness, and it was dark (ibid.).