> Of course what exists or doesn't exist in the universe that's not visible is pure unscientific speculation

If something can only be proven to a 99.999999% certainty, is that still "unscientific"? What if scientists predict something happening with only 0.1% chance, for example the likelihood of a particular asteroid hitting earth? Or what about 60%, like a typical weather report predicting rain.

Generally, the high, low, or intermediate likelihood of some event happening does not make it unscientific.

While it would be unscientific to simply say that aliens exist, it's plenty scientific to consider the rate of star formation and age, conditions for life around stars, and time it takes to develop intelligent life to say that there is a particular likelihood of there being alien life.