An argument against other intelligent life
in space is because of the Von Neumann Probe Theory
Because of the problems and limitations of interstellar travel, the creation of self-replicating probes seems the obvious solution to exploration of the galaxy.
Since we have not seen any probes reach Earth, the theory states that is because there is no intelligent (or technology creating) life out there; otherwise we would have been visited by now.
A counter argument is that we are among the first intelligent species in the galaxy.
A counter-counter argument is that since we live on the outer rim of the galaxy, we should be among the last to develop (those nearer the galatic core are under higher amounts of radiation and should mutate at a higher rate).