Actually, I don't think a spaceship would be capable of taking you to an alternate universe. It's not as if you get to the edge of space and there's a door you can step through into Alternate Universe #12, or something. You'd be better off with some sort of... device, like a teleporter, or something that could disturb the universe's, uh, structure enough to let you break through.

Sooooo, anyway. I dunno how we got here. Probably some sort of disturbance in the space/time continuity. Keep an eye on the cats and all the—interesting individuals turning up. You'll start to believe it, sooner or later.