Word for something that is very likely but not a fact

Question

I'm looking for a word to describe something that seems overwhelmingly likely to be true but that we aren't sure whether it is or not. For example "everyone will die". It seems very unlikely that there will ever be anyone who will never die since life is so fragile, but we don't know that with 100% certainty, so I would not feel well saying "The fact that everyone will die someday concerns me". Alternatively if there is no word for it, how could I phrase it differetly?

I'm on my phone and the comment button doesn't work, but to answer the first comment at time of this edit: I have no proof that everyone will die. Only statistical evidence in favour of it. And I'd rather not use the word "certain" when there's a chance of it not happening, since people might interpret it wrong. I'd rather have a clearer and less ambiguous wording.

Answer 1

The mathematical equation 1+1=2 is a logical certainty if you accept the axioms behind our mathematical model.

Taking the question in the spirit I think it's intended, saying that everyone will die could be called a theory.

1 : a plausible or scientifically acceptable general principle or body of principles offered to explain phenomena
// the wave theory of light
2 a : a belief, policy, or procedure proposed or followed as the basis of action
// her method is based on the theory that all children want to learn

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This is in the same sense that, for a long time, we took the theory of Newtonian physics as a practical fact—up until Einstein, at which point his theories became newly accepted practical facts.

Having said that, it would be unusual to say that anybody doesn't take mortality as a fact. However, in principle, if somebody were to unexpectedly encounter a situation where somebody didn't die, then we'd have to revise our beliefs and call our previously accepted facts of mortality erroneous. (And work with a newly accepted theoretical model—or practical set of facts.)

But any theory that is held with a certain degree of belief or confidence can be considered to be a fact as far as our grammar is concerned. Given our existing medical knowledge and the overwhelming statistical evidence of the inevitability of the end of life, death is a fact inasmuch as that word can ever be applied to anything.