I tend to define 'physical' as anything that interacts with something previously known to be physical. Then, start with something like a chair that all agree is physical.This thread has suffered from an ambiguity problem that a few posters have tried to disambiguate to no avail. The ambiguity is the result of referring to "physicalism" rather than "philosophical physicalism," or to have a clear concept of what the philosophical physicalists assert is true.
As a result of this failure to clarify terms, methodological physicalism has been the subject of several of the posts, an idea that needs no defense. Even the supernaturalists accept that idea, which says basically that science can only study the physical.
The position of the philosophical physicalist is that nothing exists except the physical, which generally means energy, matter, force, space and time - an unsupportable claim even if correct. Something can be a true belief, but until confirmed, is not knowledge.
The problem is that the definition of 'matter' is, at best, tricky. Is a photon matter? How about a gluon? A Higg's particle? And, more importantly, why would they be?
One can suspect that only the physical exists, and argue effectively that there is no evidence to the contrary, but neither of those is the same as asserting positively that nothing but the physical exists. The latter is a claim of knowledge that the claimant cannot possibly possess.
And here we get to the question of what it means to exist. To exist is to interact. To interact with the physical is to *be* physical. So what else *can* exist?
As for the evidence requested to support that claim, I wouldn't know what such evidence would look like. Evidence of the physical doesn't help. I would need evidence that nothing else exists.
But I'd go farther. I have no idea what evidence for something other than physical would look like. if it is detectable, it is almost by definition physical.
I'm as close as you are likely to get. I think that once we know everything physical, we know everything.Also, my guess is that there are no philosophical physicalists posting on this thread. I don't recall anybody making that claim or trying to defend that position.