“As far as we know, only machines could last the time it would take travel interstellar... “
That’s where I’m at. Certainly the odds of a UFO based on a probe is much higher than one involving alien life.
I believe the distances are so great that alien life generally stays within their star system. It is less resource intensive to modify what’s needed within a star system than to attempt to migrate to another star system. However they may send probes. The probes may still be quite dangerous depending on how these aliens think.
I also think the probability of alien life moving from one galaxy to another is about zero.
I suspect that there is a way to travel faster than light (hyperspace jumps, wormholes slipstream, wormholes, warpdrives, etc). We are still a very primitive species and we might come up with a completely novel idea.
Add quantum entanglement for instantaneous communication across the light years, artificial gravity, and finally inertial dampeners and you got a fully capable star crossing species. Scientists have theorized that all of the above technology is plausible. We just don’t understand it yet. Give us ten thousand years.