Learn German: Ich vermute, dass das Gerücht doch wahr ist. - I suspect that the rumor is true after all.

Here, doch translates to "after all". Modal particles add emotional flavor to a sentence. In this case, doch shows that the truth of the rumor goes against what was previously expected or said. Without it, the sentence is a simple statement: "I suspect that the rumor is true."

Why is the verb ist at the very end?

The word dass (that) is a subordinating conjunction. It introduces a dependent clause, which forces the conjugated verb (ist) to be pushed to the very end of that clause.

Why are there two "das" words right next to each other (dass das)?

They sound identical but have entirely different jobs! The first one, dass (with a double 's'), is the connecting word meaning "that" (I suspect that...). The second one, das (with a single 's'), is the neuter article "the" belonging to das Gerücht (the rumor).

Does vermuten imply something negative, like suspecting a crime?

Not necessarily! While it translates to "suspect" here, vermuten is often neutral and simply means "to assume", "to guess", or "to presume" when you think something is likely but don't have hard proof.