My friend once had this rather disturbing theory, & I'll try to explain now that I'm p***** so it'll make more sense:

Errmm,

Something along different lines of time (Can't remember this bit) but between each line of time there is enough time for a civilisation to have deveopled so far - they can reach every planet / star whatever in the galaxy. Hence another race has already discovered us. The fact that we haven't seen / discovered them suggests that they put us here & we r hence an experiment or something.

My friend was a bit strange & space generally bores me.

Mike