So the thing is, like other commenters have said, you're asking metaphysics things through the prism of science, which does not work because by nature, science uses the (mostly) objective scientific method, while metaphysics is based on subjective assessments.

You have to separate the physical, material universe as being in the domain of what can be known, from the rest, which can not be, and never will. This does not mean it doesn't exist, just that it can never be studied or proved in any way, so anyone can believe what they wish about it without leaving rationality (as long as the belief does not imply things concerning the material universe)

Umm, I thought there was the concept of antimatter. No?

Any reason to doubt physicalism?

Describe "doubt" in purely physical terms.

Any reason to doubt physicalism? Is there anything in science that says “Huh well that seems to not have any basis in the physical at all and yet it exists”

If it had no basis in the physical, then what would it mean to say that it "exists?" How you define "existence" is a very big philosophical question. Excuse me while I nerd the fuck out about something.

Physics tells us that the observable universe is 93 billion light years in diameter. However, we can sometimes observe objects leaving the observable universe. This is because of complicated physics reasons:

Physical space is expanding with time. Everything is getting farther apart from everything else, and the more distance there is between two points, the faster the space in between them is expanding. At a sufficiently large distance, the rate at which the distance between the two points is increasing, is faster than the speed of light. Neither point is actually moving faster than the speed of light, it's only the space between them that is expanding. This might be hard to understand, but think of it as if you drew two dots on a balloon and then inflated it.

Once an object gets far enough away from us that the space between is expanding faster than the speed of light, it becomes impossible for us to make any further observations about that thing. This is actually what defines the bounds of "the observable universe."

So, what happens to objects that leave the observable universe? Strictly speaking, it's impossible to say. Intuitively, we would expect that they're still there doing their thing and obeying the same physical laws as when we could observe them. But, if you told me that the stars simply vanish, or that they magically transform into butterflies as soon as they leave, there's no evidence that anyone could ever produce that would falsify that belief, because, by definition, there is no way to observe what happens outside of the observable universe. If we are defining what exists based on what is physically observable, then it follows that things outside the observable universe do not exist, even if it really seems like they should.

My conclusion from this line of thought is that existence is a relational property. I am not prepared to reject the idea that a thing has to be in some way observable in order to exist, but in that case, nothing can exist in isolation. Because for a thing to be observable means that there must exist a being which could observe it. This could be said to contradict physicalism, because physicalism would say that the material world exists regardless of our senses. I would say that the physical world only exists so long as there are beings capable of sensing it, and, should all sentient beings ever become extinct, the physical world would no longer exist in any meaningful sense.

I think the framing of questions like this assumes that there are certain “physical” things that follow one intrinsic set of laws, and certain other things that follow a fundamentally different, incommensurate set of laws.

But we don’t actually have direct knowledge of any intrinsic laws, physical or otherwise—the best we have are a set of purely provisional laws we’ve made up and regularly revise on the basis of cumulative evidence. And our method for revising these provisional laws requires that any new evidence that contradicts a law, invalidates it—provisional laws must apply to everything without exception. If we give ourselves the out that contradictory evidence can be attributed to “non-physical” causes, we can never invalidate anything nor update our models. So dualistic models are inherently unscientific—not because they’re wrong, but because starting with such assumptions is incompatible with the scientific method.