Not an assumption?.. Its a story, a tale, science fiction, which could have a little truth in it.. the way most fiction does..
"It is absurd for the Evolutionist to complain that it is unthinkable for an admittedly unthinkable God to make everything out of nothing, and then pretend that it is more thinkable that nothing should turn itself into everything."-G.K. Chesterton
"Truth, of course, must of necessity be stranger than fiction, for we have made fiction to suit ourselves."-G.K. Chesterton