Rare Earth hypothesis and the various possible filters that would have to hurdled in order for intelligent communicable life to develop. Kinda longish. Half hour. Very interesting. Despite the narration by Elmer Fudd. (seriously does anyone know what might cause this kind of speech impediment? Not making fun of the narrator, who is obviously very intelligent. It just makes the narration harder to understand.)

https://www.youtube.com/watch?v=oIva_60l3ww

As is known, there are so many unknowns that making any accurate determination is almost impossible. Huge range of possibilities. But this explanation does illustrate how, despite the trillions of stars and probably even greater number of planets, life in the universe could be exceedingly rare, or indeed even just here.

P.S. we don't have a general Science category. That would be nice to have.