So many solar systems out there, life evolved in many planets for sure. No proof but no doubt.
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There's still doubt:
https://en.wikipedia.org/wiki/Drake_equation#Range_of_result...
To prove, we would need to find such life.
To not have doubt, we need to have a reasonably high confidence that such life is there. However, the estimates are so wild and range from very unlikely to no-doubt. Thus, there is doubt (to the best we understand).
If the calculations were to say there's very high chances of the universe teeming with life at many places, but life is not 'found' yet, then I would say it like 'no proof but no doubt'.