Objects only observable outside human light spectrum?

Pretty much all solid objects can be seen within the human visible spectrum. The exceptions are frozen clear liquids, eg ice, which still often give themselves away by the way they reflect light and hold impurities. Glass can be particularly hard to see - not sure if it counts as a solid or liquid these days but it needs careful manufacturing to achieve good transparency.

Many liquids are essentially clear (effectively invisible) but again they often make themselves known by the way they reflect and refract light, particularly at boundaries.

Then there are many invisible gasses. And note these are transparent to generally any frequency of electromagnetic radiation .

It stands to reason that humans have evolved to see all possible solid objects and that the things we can't see are transparent to any light.

We can't see electric, magnetic or gravitational fields but these are not objects in that sense.