“I haven’t seen a single shred of evidence…”
“We don’t know of any evidence…”
“Nobody knew that…”
“Fox News is not aware of any evidence…”
Be on the lookout for this kind of construction, which, as charges and counter-charges fly back and forth, we are likely to hear frequently. The remark is not necessarily a fallacy, and it’s not necessarily a bad thing to admit your ignorance (that’s called intellectual humility). But the construction often leads to a fallacy, and it’s one that can be quite persuasive. When this construction is presented as a reason to think something does not exist, we start to run into trouble.
You can’t claim that x is false because there isn’t any evidence (that you know of) to show x is true. This is known as argumentum ad ignorantiam or the argument from ignorance.
“I’ve never seen a shred of evidence that smoking causes lung cancer.”
OK. You’re simply claiming there’s no evidence you know about.
“I don’t know of any evidence that smoking causes cancer; therefore, the cancer must be caused by something else” Or, “Therefore, smoking is safe.”
Here the construction leads into a fallacious claim: it can’t be true because I don’t know of any evidence.
This is akin to saying there are no sharks in the ocean because I’ve never seen one. Things do not need to be in range of your sensory apparatus in order for them to exist. The furniture is still in there even though you have turned out the lights and locked the door. And it’s dancing, throwing confetti, and wearing party hats. So don’t argue from ignorance.