I will summarize some of the existing answers, and show how they deal with different questions form the verses and commentaries.
See here Where a large number of Geonim, Rishonim, and Acharonim are cited who write that darkness is the mere absence of light. That being the case, how do we understand the plague of darkness.
Simple, first off, God could could have simply blocked light from entering the Egyptian houses.
Alternatively, the plague consisted of a fog that blocked light. Ibn Ezra recounts having experienced such fog himself.
Alternatively, one can suggest that the Egyptians were struck by cataracts as does the Torah Temimah, following R. Naftali Hertz Wessely.
However, what is one to do with the fact that no one rose for three days (Exodus 10:23)? Doesn't this mean that they were paralyzed by magic fog?
Nope. Rav Saadya Gaon explains מתחתיו to mean "from their places", rather than "from their undersides". Accordingly, the verse merely means that they were confined to their houses due to the darkness.
Thus, Rashbam and Ibn Ezra explain that 'מתחתיו' refers to their houses, and this is also the implication of Ramban.
But doesn't the Midrash interpret מתחתיו to mean the very place in which they were sitting?
Simple reject that Midrash, as it does not refer to the simple reading of the text, but rather to a fanciful aggadic interpretation.
Alternatively, the Midrash Sekhel Tov, follows the Midrashic interpretation of מתחתיו yet nevertheless understands that they merely chose to remain seated so as to not injure themselves by attempting to move around in the dark.
But if it wasn't miracle fog then why did they not light torches? The Ibn Ezra and Ramban explain that the fog was too dense for that to be effective. Alternatively, we could explain that they did light torches, but this still pretty much kept them stuck inside as lighting would have been too scarce outside. It is difficult for the modern mind accustomed to ubiquitous artificial lighting to appreciate it, but until recent times, most outdoor activity ceased at nightfall.
But doesn't the Midrash interpret them as having been paralyzed?
Answer: as already noted this Midrash seems pretty much ignored by the pashtanim, those who explain the simplest explanation of the text. Presumably this is because it adds extraneous miraculous elements, and introduces problems, such as, why didn't many or most of the Egyptians die of dehydration?
But how do we understand Exodus (10:21) which states וימש חושך? Doesn't this mean this word connote feeling; i.e. that the darkness was palpable?
Nope. Rashi and Rashbam explain that the word comes from אמש, a word for night. In context it means darkening.
Alternatively, Onkelos followed by Rav Saadya Gaon explain the verb as being related to מש; remove. The verse describes the time of the onset of the plague; the time that night is usually removed, i.e. the end of the night.
But doesn't the Midrash interpret it as feeling?
Answer: Reject that Midrash, for as we have seen, that is not the understanding of the pashtanim. Alternatively, Shadal notes that the Targum Yerushalmi interprets the noun as meaning feeling, but nevertheless does not explain that the darkness was felt. Rather, that the Egyptians groped in the dark.