Learn Russian: Это не исключение, это факт. - This is not an exception, this is a fact.

When you use "this is" to introduce or point something out, you always use the fixed neuter pronoun это, regardless of the noun's gender. If you used этот, it would act as an adjective meaning "this fact" (as in, Этот факт интересный — "This fact is interesting"), rather than the complete clause "This is a fact."

Why isn't there a dash between это and the nouns, since the verb "to be" is omitted?

In Russian, you often use a dash when linking two nouns in the present tense without a verb (like Мой брат — врач). However, there is a specific punctuation rule: when the subject of the sentence is the demonstrative pronoun это, the dash is usually omitted.

What verb does the noun исключение come from?

It comes from the verb исключать (imperfective) or исключить (perfective), which means "to exclude." An исключение (exception) is literally something that has been excluded from the general rule.