The specifics on how it is sentient is unknown. We do know that people can be absorbed into the Domain, and that it has the remnants of personalities and consciousness of those dead. .

Not just the dead, but everybody who has ever visited it and those who [i]will[/i] visit it. Remember the Ur-Didact says in Silentium that he has heard Forerunner descendants? [quote]"The Domain can only tell the living what they already know," he says. "Or what they've stored in its expanses. I wandered through all the corridors... so they appeared, anyway. Centuries of wandering through hallways and caverns and even deeper, darker places, lined and fitfully aglow with ancestral records and memories, upwellings of past visits, rarely by me, sometimes by our ancestors... on occasion, our descendants." "Descendants?" I ask. "The Domain keeps its secrets only with difficulty. It wants, it [i]needs[/i], to spread knowledge. It wants to tell us when we're being foolish, but it can only replay the emotions and memories of those who came before. Still, rarely, it violates its own rules." "What about our descendants?" I persist. "I felt their touch, their love. And yet, they were fading. The Domain is filled with sadness. A deep shadow has fallen over everything Forerunner."[/quote]