Re:Why We Haven’t Met Any Aliens
by asdkl234890 on Thursday December 22, 2011 @01:37AM
(#3380667)
Also space is really BIG, and no one can go faster than light.
BIG space doesn't solve the exponentially-reproducing colonist issue though...
Plus, if an exponentially-reproducing civilization appeared even in a neighboring galaxy a couple million years ago, we'd probably be able to notice something very strange.
If it appeared in this galaxy, we'd probably never have evolved.
But as for your list I think you left out the Occam's razor option:
5) Rare earth hypothesis
http://en.wikipedia.org/wiki/Rare_Earth_hypothesis