I think that like the atoms of the material world thought is also a stuff of the universe, existent. When we die our consciousness enters this global thought domain, the non physical domain of the universe. Total knowledge is there so ofcourse we know of our own past existence, as we know of everything that transpired within the universe, but now we are a part of the global domain of thought. We know of everyone’s existence as we merge with total knowledge, universal knowledge. You might say we cease to exist but we always have existed and always will. Time is not apparent there, existence is the thing.