Perhaps you need an example of why what you call an argument from ignorance is no such thing. It is a null hypothosis.
No.
"As an example scenario is of a man sitting in a warehouse with a tin roof and when he hears no sound of raindrops, he assumes that it is not raining, without looking outside for any evidence of rain.[5] Here ignorance about a particular form of evidence for rain (the noise) is used to assume a lack of rain; but the conclusion may fail if it is raining so softly that no noise is heard by the man.[5]"
http://en.wikipedia.org/wiki/Argument_from_ignorance
The man in the warehouse might as well be the posters here claiming "there's no evidence of aliens, so we should assume they don't exist".
Also: "
science is saying in the absence of evidence, we must withhold judgment"- Carl Sagan.
For the sake of argument let us assume you are right and the default possition is not to assume something does not exist until you have either evidence or a compelling reason to believe otherwise, and that any claim "x does not exist just because I don't have any evidence" is an argument from ignorance.
LOL, for the sake of argument? Seriously?
I will make three statements. Disprove them with out relying on what you call an argument from ignorance:
1) "Ghosts" are fleeting glimpses into alternate realities.
We've done exhaustive searches for ghosts and paranormal activities and come up empty every time. Also, claims of ghost sightings are never reproducible and often fraudulent. Therefore, we have good reason to reject the existence of ghosts.
2) At least three users of this forum are aspects of a very complex computer programme, that is capable of falsifying Ip addresses, ISP records and even CCTV footage, receipts, and all possible digital records of a "real" person so any investigation you make short of meeting the person will produce all the evidence you would expect of a human.
Are the programs doing the postings? If so, the claim can be rejected because that would exceed the limits of where computers are at, at the moment.
However, I wouldn't put it past someone to create just such a program as the one you describe
http://en.wikipedia.org/wiki/Computer_poker_players
If you claimed that Tsig was one of the programs in question, I would actually find that somewhat believable.
Angels exist. They serve the creator of the universe in a form that does not directly connect with "reality" unless they wish to do so. They are beings of pure thought and metaphysical powers who communicate through psychic means and absolutely no form of human technology can ever detect them, therefore they can never be shown not to exist and must be assumed to exist.
That doesn't follow. At best, they "must be assumed to POSSIBLY exist".