The Marvel Cinematic Universe has woven an impressive tapestry that's grown only more complex with new concepts such as the multiverse. These new concepts have also introduced new mysteries, like what will happen to the massive Celestial in the ocean. But for each big mystery, there's also been much smaller ones that helped forge the franchise, such as who killed Howard and Maria Stark and how that impacted The Avengers.
Source：https://www.cbr.com/mcu-black-widow-never-solved-mother-mystery/
