Reylo/star wars fans Attention.
World Between Worlds Theory: Since Leia's body doesn't disappear until Ben's death, she could've somehow helped Ben get to the World Between Worlds (WBW). Keeping in mind that Exogol and Ahch To have portals to get to WBW, providing a feasible way of getting there.
So Ben is stuck there til Rey frees him, or he is able to free himself (this is why his force ghost doesn't show at the end of TROS)
In WBW, time travel is possible! I believe Ben was the shadowy figure in the mirror. Luke even said she was drawn to the dark. Rey was drawn to Ben.
This absolutely blows my mind!!