> his is really only true if you have a firm grasp on the space you would need to search to find evidence, and that you’ve done the work looking and come up empty

Only if you're making definite proclamations, which I'm not. I'm not claiming complete knowledge here, simply that there's been no evidence presented that supports the belief in such a thing, so I don't hold belief in one. There's no assumption there. To dip into physics as you seem so keen on bringing it up - there's no evidence that dark matter is really the corpses of ancient aliens, so that's not a working hypothesis for me either. I don't know what it is, I'm not sure anyone does, until we hold evidence I am a-it's-the-corpses-of-ancient-aliens-ist. I don't hold that positive belief, and I would find it weird if people came along and told me that I was making strong claims in the other direction if I said I didn't believe it.

> How would you propose we go looking for afterlifes

Most appear to be unfalsifiable propositions, and as such it seems a pointless quest. But I don't need to go looking in an exhaustive fashion to hold the position that I don't believe in one, given current evidence, because I'm not making an assertion that there is definitely not an afterlife, or that there cannot be one. It is sufficient that there's no evidence for one, so I don't hold the positive belief that there is one.

Could there be some sort of afterlife? Sure, conceptually, but so far there's no reason to think there is one, any more than there's reason to believe in fire-breathing dragons or invisible unicorns.

> By claiming that “available evidence” (ie none) informs your belief that there’s no afterlife

I don't have "a belief that there's no afterlife". There's the flaw in your thinking.

Thought experiment - I claim that that I can travel to another planet, instantaneously, and live a year of my life there because time works differently, and then travel back, all in the time of a blink of the eye. I just did it now! Anyone can do it, in fact lot's of people do! Do you believe me? No? How do you know I didn't do that? You're making a positive assertion about the nature of the universe there! Your belief that I can't do that is unevidenced!

Or... you can say that since there's no evidence, that seems pretty weird and unlikely, I'm going to go with the working assumption that humans can't do that until such time as some sort of evidence is presented. This is atheism, to me.

> This is by definition, an oxymoron

First line of the wikipedia link on atheism - "Atheism, in the broadest sense, is an absence of belief in the existence of deities"

First line from the one on agnosticism - "Agnosticism is the view or belief that the existence of God, of the divine or the supernatural is unknown or unknowable."

These do not contradict one another. I don't claim concrete knowledge, so in the absence of evidence I hold no positive religious beliefs. If the existence of the divine is either unknown or unknowable then there's no reason to give it much thought. I don't believe in it. Which is distinct from "I believe it is impossible" or "I believe it is definitely not present", positions that you seem to want to force on me.

> There’s no need to be condescending

I'm not convinced - when someone is busy labouring to create a straw man and to tell you what you mean by your own words, I think it's probably an apposite response.