As I understand 'those' here are 4% from the 1,000 participants. Am I right? If so why do they add 'under dark skies'? Is it necessary to understand why are 'those'? What a part of speech is 'under dark skies'?
According to the results from the 2014 Star Count, 59 percent of the 1,000 participants could see 10 stars or fewer within the four corners, an indicator of severe light pollution. By comparison, those under dark skies — a number that amounted to only 4 percent of participants — could pick out 30 or more stars.
Article - England launches 'cosmic census' to combat light pollution https://www.mnn.com/earth-matters/wilderness-resources/blogs/england-cosmic-census-combat-light-pollution