Anything except God.
Naturalists and atheists will propose any theory no matter how outlandish to avoid having to admit the existence of God.
Is the universe a massive computer simulation created by aliens?
Is the universe just one instantiation of an infinite number of other universes in a much larger but undetectable multiverse?
Is the universe one massive, integrated consciousness that controls itself?
Is the universe just the disembodied intelligence of advanced alien life forms?
Is the universe the creation of an All-Powerful Divine Creator?
OMG, how irrational? How silly? Is this the stone age? Do you believe in fairy tales?
Why are the world’s leading scientists proposing these farfetched theories in the first place?
Because they realize that a pure physicalist explanation of the world is not adequate. They realize that a purely naturalistic picture can’t answer the biggest, most pressing questions. They recognize that the universe has all the appearances of being designed, created as if there is some intentionality and will behind it all. But they refuse to admit that what they really have in mind is God.
So they have to come up with aliens, computer simulations, and other just-so stories to make sense of what is nonsensical as far as strict materialism is concerned.
Maybe one day they will unbury the truth that they have worked so hard to cover.