I have discussed this to some extent in the post “Pāṭihāriya (Supernormal Abilities) of a Buddha – Part I.”
- More relevant posts in “Buddhahood Associated Controversies.”
The answer to your question is partly in your question: “So when Ven Moggallana went to find the end of the universe, how did he go? Was it like a photon type of massless state?”
- The manomaya kaya (gandhabba kaya) that Ven Moggallana traveled with is almost like a photon; it gas only a few suddhatthaka. A photon has a zero rest mass, meaning it is “all energy.” See “The Origin of Matter – Suddhāṭṭhaka.” P.S. The transition from pure energy to suddhatthaka is evident in the chart made by Dosakkhayo; see #5 of “Bhūta and Yathābhūta – What Do They Really Mean.”
- A suddhatthaka is just above that “zero energy state.” Thus, a manomaya kaya can approach the speed of light. That is how Ven Moggallana (or the Buddha and others) traveled at such high speeds.
- The “body” of a Brahma is also made with a few suddhatthaka. They also can travel close to the speed of light. When the Buddha attained Buddhahood and saw how hard it would be to teach his “newfound Dhamma” to humans, he was slightly discouraged. Brahma Sahamapati (an Anagami) read Buddha’s mind and came to visit the Buddha to remind him that there are enough humans who are capable of comprehending the profound Dhamma.