The Absence of Evidence
See Nonsense On Stilts.
The best way I can think of to describe the difference is the following:
If you wanted to determine whether someone was in a room, and someone poked their head through the doorway but kept their eyes closed, then reported that they didn’t see anybody, you wouldn’t take that as evidence that there was no one in there. That’s not evidence, that’s failing to check for evidence.
If someone poked in and looked around, and then said there was nobody there, that would be evidence that there wasn’t anyone there – in other words, evidence of absence. Or alternatively, evidence against presence.
The other way is the absence of evidence. It doesn’t mean anything at all.
If you haven’t made any observations, you have no positive evidence to support positions. That is not evidence against a particular position, just a reason not to assert one. To actively exclude a position, you need evidence against it.