We cannot percieve them because they were not part of these set conditions that moulded us. — Susu

How do we know about the strong or weak nuclear force?

Phlogiston? Aether?

A "physical property" is, first of all, a theoretical concept. Different theories posit or imply different physical properties. So in one sense, one can quite easily imagine a physical property that did not exist before: just assume a historical stance and consider, for instance, that temperature was not a "thing" before it was made part of 19th century thermodynamics, and even then different theories defined and operationalized in different, sometimes contradictory ways. Other theories do not even include temperature in their ontology of properties. Future theories, such as theories of quantum gravity or perhaps new theories in climate or materials sciences, may introduce heretofore unknown physical properties.

Flip this property/theory relationship, and you can readily imagine physical properties that do not exist even in principle. One end of a scientific theory is an abstract construct - the other end is supposed to be anchored in reality. Lift that anchor and use your imagination freely to construct a theory that does not seek to model anything in reality. Chances are, this theory will have nonexistent physical properties built into it.

Negative mass.

Imaginable and possible

Not the same sort of thing.

Superstrings, branes, parallel universes, wormholes, singularities, gravitons, pilot waves and any other postulated physical entity that lacks empirical validation.

There isn't such a thing as 'physical phenomena', for no physical concept can be given a persistent interpretation in terms of experiential phenomena, as the logical positivists soon realised.

Take for instance Red. It's physical meaning is defined in terms of optical wavelength, not in terms of a sensory impression, and we cannot give an exhaustive description of the context that is necessary such that an application of optical red always produces a "red" sensory impression. Furthermore, the theoretical meaning of "optical wavelength" is holistic, given that it is a property whose meaning and experimental determination rests upon the understanding and application of the rest of physics. Physical red therefore cannot be given a phenomenal definition.

We use our sensory impressions as estimators of our use of physical terms, but they aren't the definitions of our physical terms. On the other hand, we shouldn't forget that our use of physical terms is ultimately determined by the totality of our sensory impressions. So we aren't justified in jumping from semantic holism to physical realism.