Brainiac 5's the second smartest guy I know. If he can make a machine to view other universes, then traveling there can't be too hard. Look at what they can do already.

[They already have instant teleportation or whatever it is the Threshold Gates do. And time travel.]

Like Sam said, it's easy to believe insane things when there's nothing else there. Easier still if you've got proof, like real people telling them about it.