Theory, a bit of a spoiler:
If it turns out that the main character returns to Earth and is greeted by this universe's equivalent of the Planet of the Apes, my respect for this novel will double.
It could be time travel or the theory that is million lights year's apart so he lives in a different timeline but my personal theory is that his perception of time was messed up or he was abducted for quite some time now by aliens soknos or other organizations and just left them there and he had no memory of it