WARNING
*AVENGERS: INFINITY WAR SPOILERS*
Oh yeah! Sorry for the later response I had practice. Anyways...my theory is that everyone who "disappeared" was sent or was already in a parallel universe and in that universe it was opposite (everyone who didn't vanish vanished). Then, in the next movie, Thanos takes over and kills all the old avengers, leaving it up to the other universe--which will become the NEW MCU--to stop him. OR...when Doctor Strange said "it was the only way" to Tony Stark, after he gave Thanos the time stone, he was referenced the 1 out of 14,000,601 times that Doctor Strange saw the Avengers beating Thanos. Maybe--just maybe--Doctor Strange was protecting Stark because he was the key to success. Another thing that I realized when I was checking out some Marvel stuff is that when half the universe vanishes, they are housed inside Thanos's infinity stones, much like a coma...so maybe he has the power to let them out as well as keep them in. Anyways, that's my theory, thanks for reading if you made it this far without getting bored! @gladysObando