What we know about mathematics can't prove or disprove things we simply don't have any idea of. Think as if other beings would live in a different frequency plane (outside of our 3 spacial + 1 time), our instruments and theorems can't detect that.
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).