Talk to Zavala
Speak to Zavala in the Hall of Guardians at the Tower.
"A Warlock asked me once why the Void forms as a shield around a Titan. He went on about seeing into the abyss, the absence of form, that sort of thing. I waited for him to stop talking, passive.
After his mouth wound down, I just nodded at him. 'That's why,' I said."