Crash you are bring the other thread into this one. This is more (or should be) of a theoretical thread to discuss the nature of evidence. If the idea of "absent evidence" being useful is clarified properly here then it's misuse can be prevented in other threads.
In parts, at least, of the discussion here the absence of evidence is not simply "ignorance". It is a collection of null results from selected experiments. Enough of these, in my mind, may be considered a form of evidence, even useful evidence.
In fact, a serious problem in research today (certainly in medicine) is that null results tend not to be published. These experiments didn't "find the keys". These results *are* evidence and the tendancy not to publish them results in misleading meta-studies.
What you might want to sort out is what is required to make the absence of evidence into something useful. Clearly if no work is done what so ever (either because we haven't or can't do it - eg. we can't look at other universes yet) then the absence may be considered to be meaningless. However, there must be a point at which continuted absence starts to become meaningful.
If it takes 100,000 "looks" to search the experimental space we only have proof of absense when we have looked at all 100,000. However, I am not going to be easily convinced that after 99,999 looks we don't have something that we can draw a pretty firm tentative conclusion on. If that is now meaningful how about 99,000?