The most common objections to physicalism are

-there remains gaps in our knowledge

-we don't know what we don't know

-since we can imagine violations of it there can be violations of it.

Addressing each point:

1. Gaps in our knowledge do not mean you can insert whatever you want.
2. We haven't established that there is even something we don't know.
3. Imagination does not have veto power over the universe.