First, what is materialism? I'll keep it brief, not really going into detail but rather just giving a succinct definition (with which I have some issues).
In philosophy, materialism is the doctrine that nothing exists except matter and its movements and modifications.So immediately we see both the problem with the unreasonable demand for "positive evidence for materialism" and with the notion of materialism, as defined philosophically, itself. I'll address each of these issues in turn.
Positive evidence for materialism? An annoying dodge.
"Positive evidence for materialism" is an annoying dodge, one designed specifically to distract people from the fact that we very apparently live in a material universe and to suggest that there's anything like "positive evidence" for any alternative. All of the evidence we have, by the very definition of evidence, supports the existence of the material world. None, incidentally, supports the existence of anything non-material (or, specifically, "spiritual").
Worse, if we look at the definition of the philosophical position called materialism--that nothing exists except matter, etc.--we immediately run upon the problem that "positive evidence" is basically a demand for something impossible. To provide "positive evidence" for materialism would require us to definitively prove empirically that nothing at all, anywhere, ever, is anything but matter, its movements, and its modifications. (I trust that energy is contained within those via Einstein's famous connection between the two.) An appeal to ignorance is all that's needed to defeat any claim to "positive evidence" for materialism, so it's a useless dodge, a waste of time, to even bring it up.
Thus, I think of this demanding "positive evidence" for materialism to be more of what I have called a talisman meme than anything of any substance. When people spout talisman memes--easily repeated ideas of little or no worth waved about to ward off ideas they don't like--they're attempting to distract from productive conversations that would be destructive to things that they want to believe.
"Positive evidence for materialism" is one such talisman meme, a disingenuous call for something impossible that hopes to distract people from the fact that the material universe is all we know exists in order to try to mitigate the damage that fact does to particular cherished belief systems.
What's wrong with materialism?
Oh, philosophy, this is how you shoot yourself in the foot all the time. First, note that the philosophical definition of materialism is that it is a doctrine. How obsolete. Second, note that it applies a universal--all is matter, nothing else. How obsolete too.
This, though, is a common bane of philosophy, aiming to provide humanity with the "right" doctrines that apply universally. It is a bane for at least two reasons, both of which amount to giving people reasons to argue over things that are ultimately pointless, as demonstrated by people demanding "positive evidence for materialism." (The other is that people don't really think that way, in terms of picking and choosing particular universally applicable doctrines and remaining purely consistent with them, or at least they don't think that they do and are thus prone to ignore the doctrines when it comes to it. Of course, philosophy goes on to create even more of a public relations nightmare for itself by gleefully pointing out when people are failing to be consistent with doctrines that they think they espouse.)
The point isn't--and never should have been--to declare a doctrine that says all is material and then adhere to it. The point is that all we've ever observed is material, or can be accounted for materially (e.g. thoughts in the latter category). In other words, an assumption of materialism is provisional and thus not really an assumption that we've used to paint ourselves into a corner.
Oh, and literally all of the evidence points us toward accepting that something like materialism is very probably true. Do you see the difference? We're not aiming for a doctrine here that applies like a supertruth but rather for a best-guess about the world we live in. We don't need stupid assertions that "nothing exists but" when we can just keep doing as we're doing, looking at the world and seeing what we see, taking it for what it is, whatever it is.
I don't think it needs to be that way, though. I think our fictional accounts are fully demonstrative of innumerable ways in which non-material, by which I mean "spirit" or "magic," entities can be imagined to interact with the world in a meaningful way, a way that we could, in principle, detect and make sense of. The fact is, though, that despite a lot of looking for such, we never, ever find it. (We may, of course, but it really need not be mentioned.)
Missing the point
Materialism, then, as a philosophical position, misses the point. The point isn't to declare that we've figured out how the universe works and then dance around with our unknowable supertruth, squabbling endlessly with people who have adopted different ones. The point is that everything we see points us in a direction that's like materialism without the dogma inherent in it.
More than that, talking about materialism as though it is important to do so misses the point (this goes also for the similar "naturalism," a philosophical viewpoint according to which everything arises from natural properties and causes, and supernatural or spiritual explanations are excluded or discounted). As soon as someone brings these words up, I instantly distrust whatever they're about to say. What we (effectively, which is to say almost surely) know is that the material world exists, nature exists. We do not know more than that, and there's nothing gained by pretending that we do by asserting ourselves as adherents of doctrines like materialism or naturalism, though there's lots to be lost--these stupid arguments about whose assumptions are better are, to put it plainly, a grand waste of time and talent and an open invitation to keep talking about the world and our knowledge of it in the wrong ways.