There are interesting explanations of how the Plague of Darkness actually took place. On one hand, R’ Avraham Iben Ezra learns that it was a fog so tremendously thick that it extinguished any fire lit within it. He writes that he himself saw experienced such a phenomenon many times near the ocean. Yet the Torah Temima understands that the plague meant that the Egyptians were stricken with severe cataracts. The Vilna Goan explains that darkness is not like we commonly tend to think of as simply the absence of light, but rather a creation in its own right. Hashem however set up the light/dark relationship in such a way that light always wins in a “fight” with darkness. By this makkah, though, that relationship was reversed.
Rabbeinu Bachaiyei (Bo 10:21) seems to learn a pshat somewhere in the middle. He quotes the Medrash Shemos Rabba (14:1-3) detailing and expounding upon this plague. He mentions the tangibility of the darkness; this darkness was not just the absence of light. Rather, it was an existence in itself that had substance. So thick was it, that during the last three days of the six day duration of this plague, no Egyptian could move a muscle and was frozen in place. (Ralbag writes that Hashem sealed the Egyptians’ noses and mouths. They could not breathe for three days. That they did not die was a miracle. He did this because had the Egyptians breathed in this new, thick dark air, they surely would have died. Being kept alive without breathing for this time was a source of tremendous suffering for them.) Klal Yisrael, however, had plenty of light, not only in Goshen but even when they entered the Egyptian houses to search for valuables.n
Rabbeinu Bachaiyei explains the nature of this particular darkness. In order for the eye to see light, the light must travel from its source through the air into the eye. This is similar to hearing; the sound waves travel from the source to one’s ear. In other words, air is the medium through which light travels. During the first three days of the plague of darkness, Hashem “sealed” the pathways of the air from allowing passage of light. In the absence of the ability for light to get through the air automatically turns dark. For the last three days, Hashem thickened this dark air so much so that the weight of it did not allow them to move. This was not the case for Klal Yisrael; Hashem did not close the passageways of air for them. They were able to see freely and could go where they pleased.
In understanding this Rabbeinu Bachaiyei, it would seem that one would need to clarify his words as follows. We cannot say that all the air particles in any specific Egyptians house were sealed off to light. For if so, how could the Jew entering to search for valuables be able to see? On the other hand, to say that the air particles were open to light would mean that the Egyptians would be able to see! One must say that the plague of darkness how we tend to envision it. It wasn’t that the land of Egypt was completely dark. Rather, the air particles immediately and in closest proximity to the individual Egyptian were the ones that were sealed off from light (for the first three days, after which this very air became heavy enough to hinder any movement). It was as if every Egyptian had a heavy, dark shell around his body. But during the day, the land of Egypt itself was as bright as any other country.
One could comment, however, that according to this the Plague of Darkness effected the Jews as well. Being that the air directly surrounding the Egyptians did not allow light to pass through, all that a Jew saw in looking at an Egyptian was a thick human-shaped black cloud. The Jew would not have been able to see through due to the sealed air. If, for example, the Jew would want to know the identity of the Egyptian whose house he had entered by looking at him, he would not be able to (and those Jews who were able to tell specific Egyptians about the whereabouts of their valuables would have had to have know their identities by other means)! Possibly one could suggest that the air around the Egyptian worked like one-way glass; one side can see through while the other side can’t. The Jews could see the Egyptians while the Egyptians could not see out. The problem with this might be that if the light could not get in to the Egyptians, then it would not be reflecting back towards the Jews to enable them to see the Egyptians.
The easiest pshat in Rabbeinu Bachayei might therefore be that the air was open for the Jews and closed for the Egyptians. Though this may not make sense in our minds (as we asked above), we can safely throw up our hands and say, “Who is so wise to understand Hashem’s ways!” So writes the Alshich (10:21-23). The Ramban at the end of Parsha Bo explains that all the miracles preformed in Egypt were a testimonial for generations of there being really no such thing as nature, rather everything is Hashem’s doing. The miracles there were a wakeup call to this. After writing this, I found in the Medrash Tehilim (aka Sochar Tov 22:2) exactly this idea. “In the way the world works, can a man light a fire and say, ‘Ploni who is my friend shall benefit from this light, but Ploni who is my enemy will not’?! Rather everyone benefits together. Yet Hashem is not this way. He can shine light to one and place darkness on another.”