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But they are not 100% sure.

• jacquesm 7y ago
  That's how science works. 99.9% still leaves a small chance, so no good scientist would say 'I'm sure'. They'd only say that if they had proof of something and you can't prove a negative. So there never will be proof that it is not the case, only ever more evidence that it probably is not the case. And all of that evidence could - in theory - be made moot by proof to the contrary.