Conspiracy theorists often claim that a lack of evidence for the alleged conspiracy only goes to show just how powerful the conspiracy really is because it can suppress the evidence. Is this an argument from silence, or an appeal to ignorance?
In one sense, it feels like an appeal to ignorance, because the claim of an undetectable conspiracy is unfalsifiable. But in another sense, the precise lack of evidence is being used as an argument, so it feels like an argument from silence.
Neither. The fallacy you're looking for is begging the question. If the conspiracy theory is true, then the lack of evidence for the theory would indicate the power and reach of the conspiracy used to hide it. But the lack of evidence for the conspiracy can not reasonably be seen as a valid argument supporting the truth of the conspiracy theory. Those who strongly believe in something often have a hard time doubting it, which is what is required in order to persuade a skeptic. This is where a lot of question begging comes from, in form. If you already believe something is true, you don't generally look for evidence to convince yourself that it is true, and you may not even know how to "think like a skeptic." But of course the first thought a skeptic would have is, "Suppose the proposition is false, would anything appear different?"
