Why would the interior of the great pyramid be very dark

so it would be moist and cool

It would be dark and cold to conserve the mummies and all organic contents.

However, apparently the pyramids contained species of lamps that were primitive to energy. One explanation would be the batteries found in Baghdad, Iraq in 1938, dating from 200 BC. They have a copper cylinder surrounding an iron bar, immersed in an acidic element like vinegar inside a clay vase. Tested produced energy, but weak produced. It would be a very plausible explanation for the batteries that lit the Egyptian lamps.