In article , (Eric
Flesch) writes:

On 01 Jan 2018, Gary Harnagel wrote:
G-type stars that are gone now
but the intelligent species "hatched" by them have migrated to a younger
star. At only 0.1% the speed of light, generation ships could cross the
entire galaxy in a mere 0.1 billion years.

But they never came here, to Earth. And yet they would have spread
out over the galaxy, the same as yeast in a Petri dish. What part of
the Petri dish escapes the yeast? None. There is your evidence that
no such alien civilizations exist.

Not really. Of course, technically, one can never prove the
non-existence of something; at most, one can accumulate evidence for it.
(Similarly, one is not asked to prove one's innocence in court; rather,
the accuser must prove one's guilt.) Even so, you are making two
assumptions: that an alien civilization would want to colonize the
galaxy, and that they would have no reason to avoid us. The latter is
known as the "zoo hypothesis". With regard to the former, remember
that an alien civilization is ALIEN. They might have very different
motivations. (Even so, since it requires just ONE alien civilization,
or actually just one (sufficiently powerful) individual, it doesn't seem
unlikely that such colonization would take place. However, the zoo
hypothesis does seem to make sense.)