Why do moon jellyfish glow in the dark?

My guess: it is because this type of jellyfish live in the very bottom of the ocean, so over time the evolution of that species needed to get around im the dark, so hence the moon jellyfish glowing.

The fact: The reason why jellyfish glow because of a chemical reaction that causes light to be made inside the body of the jellyfish. If they suspect a predator, they glow to statle the predators, it is their way of defense mechanism. This whole process is called boluminescence. Even some species of squids can do this exact thing!