[Dog Gone]

Every civilization capable of reaching the stars is capable of blowing itself up or being hit by some space debris that effectively ends civilization or even the species entirely.

Alternatively, any civilization that reaches a suitable planet may discover that any life existing there is extremely toxic to their systems. Our best defense against a colonizing life form could be E. coli, for example.

[DentsRun]

Every civilization capable of reaching the stars is capable of blowing itself up or being hit by some space debris that effectively ends civilization or even the species entirely.

That's what I've always thought. Any civilization that stays around long enough to develop travel between the stars will probably get hit by a civilization-destroying asteroid before that every happens.