כי הנה החשך יכסה ארץ ... ועליך יזרח ה' ... (ישעי' ס-ב)
While Yeshaya HaNavi proclaims that the light has dawned and the presence of Hashem has finally returned to Klal Yisroel, his very next words depict a gloomy picture, saying, “Behold! Darkness shall cover the earth ...” It is noted that prior to the coming of Moshiach, great darkness will descend on the earth covering Klal Yisroel - and by extension the nations of the world. However, from that darkness, “Upon you [Klal Yisroel] Hashem will shine, and Hashem’s presence will be seen...” The question is, why indeed is this the way it will be? Surely light emanating from light will have a profound impact as well?
R’ Ahron HaDarshan zt”l (Bigdei Ahron) explains that from the words of the Mekubalim when we say "גולל אור מפני חשך וחשך מפני אור" - light and darkness are intrinsically tied to each other, and just as there is no light when there is darkness, there is no darkness when there is light. This is the way of the world - sadly, when one element has completed its task and another takes its place, the first element is quickly forgotten.
He explains that right before sunset the sky is filled with a bright light after which the world is plunged into darkness - dispelling the foolish notion that He Who creates light cannot create darkness. Thus, while Yeshaya’s message notes a great surge of light followed by darkness, as soon as the moment of redemption is reached, the light will once again take the place of darkness - proving to all that Hashem pulls the levers, and light can shine just as darkness descends.