Learn English – Need a word that means “the worst that can happen”

A fact does, in fact, have to be the truth at the time you're using the word.

By 'truth', I mean something you believe to be true (due to any of several possible reasons).¹

Consider: "The number of planets in the solar system is eight." A few years ago, this was not a fact. It is now. (Just an example, don't attack the example.)

[EDIT: Before 1917, people thought it to be a fact that the atom was the smallest particle of matter. Today, it is a fact that it isn't, and we were wrong in thinking it was... I'm including this example to show that in light of the new fact, what we thought was a fact previously, can be rendered untrue for that time too; something @Jay pointed out my previous example didn't specify]

When a jury convicts a man, it's a fact that he's guilty. If he's later acquitted, it's a fact that he is 'not guilty'. (As far as the public is concerned. Individuals who actually saw the crime might know, for a fact, whether it's true or not)

What I'm trying to say is that the word fact is used for what you know (or sincerely believe) to be the truth or what is widely believed to be the truth at the time of speaking. Facts are subject to change.

Something is not a fact if you know/believe it to be untrue or if it can be easily be shown to not be widely believed at the time.

The two can contradict. That's when myth comes into play.

Tom: "Interesting fact: you can see the Great Wall of China from space." (Widely believed).

Neil: "As a matter of fact, you can't. That's a myth." (I went to space. I know better.)

¹ _{Please note that I'm talking exclusively about the cases where you use the word fact. In those cases, I infer you strongly believe it to be true.}

If you have no specific potential doubt in mind then it's 'as certain as blow it'. But that may be too casual/informal for your purpose.

If something is 99.999.... certain, then it is as 'near certain as makes no matter'. but if the difference makes no matter, then you have no reason to doubt it at all. And in that case, you may as well say it is certain.

The point 0.0000000...... chance of being wrong is so small that you never get to a 1 at the end. If if such an improbability were to occur, nobody could possibly calculate its probability. It would be like the probability of all the atoms in the chair you are sitting on moving upwards at the same instant. We are talking about the infinite improbability drive in 'A Hitch-hiker's Guide to the Galaxy. After all. 0.00000.... is infinite improbability.

Short of that, 'nearly', 'almost' or 'virtually' certain are all fine. In real life, if the virtually impossible does happen, it will not help you to plead that in fact you were right on the grounds that it it was, in fact, virtually impossible, unless there is a precisely calculable probability. In which case, I guarantee it will be at least 1% and probably more.