One way life can get from star to star is freeze-dried, floating on rubbish. At non-relativistic velocities, this takes millions of years, so there isn't much commercial potential there. A number of lifeforms here have the ability to shut down when times are harsh, and start back up when circumstances change. Viruses go inert. Some protozoans sporulate; that is, they form a dried-out egg with zero metabolism. On adding water, the daughter creatures swim away. I'm not particularly promoting panspermia here, just recapping. One reason why a species might travel would be if it foresaw its own demise-- no choice, migrate or die! If they couldn't solve the Einstein riddles, they'd find a way for something of themselves to survive the non-relativistic trip. Eggs in stasis, frozen germ cells tended by robots-- whatever.

Pardon my rambling. I have personally opened up a bit of millions-of-years-old shale, and found the fern still green inside. The fern didn't survive, of course, but what about its microscopic parasites? Sometimes the forces that drive a migration have little to do with choice. And the thing we need to keep foremost in mind when considering the possibility of alien life is that it might be... alien. I don't have any of the evidence you seek of actual, real-life aliens, unless we are they, but that's philosophy, not hard science. Steve

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Ignoramus et ignorabamus.-- Reymond

Wir mussen wissen. Wir werden wissen.--Hilbert

Pick one.