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This includes all forms of viewing: telescopes, binoculars, naked eye...

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    $\begingroup$ Where: just In our solar system, orbiting stars in the vicinity of our solar system, rogue planets that aren't orbiting any star, objets anywhere in the galaxy, objects anywhere in the observable universe, or objects anywhere in the universe? With what equipment: present-day technology only, or telescopes under development, planned for development, or just dreamed of? This question is a bit vague. $\endgroup$
    – David Hammen
    Commented Dec 1 at 8:40
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    $\begingroup$ You may e.g. want to ask about what is (with current technology) unobservable. But asking about what is unseen in this general way without any limits to the question is too broad. $\endgroup$
    – planetmaker
    Commented Dec 1 at 9:57

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There are no such solar system bodies

There are a few small objects that have been detected by probes, but not seen by Earth-based telescopes, these might include some moonlets embedded in Saturn's rings, or individual rocks on the surface of Mars. But none of these is gravitationally rounded.

To be rounded, a body needs to be at least about 400 km across. Moons of this size are easily seen at the distance of Pluto (with a professional telescope), and all dwarf planets and large moons that we know about have been discovered from Earth. There are many small moons that have been discovered by space probes, but they are all too small to be gravitationally rounded.

However, many exoplanets can't be seen from Earth. Their existence is deduced from changes in their star's light, as the exoplanet passes in front of the star, or causes the star to move. These changes can be detected from Earth, but the actual exoplanet can't be seen.

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    $\begingroup$ Regarding all dwarf planets and large moons have been discovered from Earth -- are you sure of that? Professional solar system astronomers are not. One of the goals of the Vera Rubin Observatory (first light in late 2025) is to find ever more Kuiper Belt Objects, some of which are certainly larger than the potato radius, and to assist in the search for Planet Nine, if it exists. $\endgroup$
    – David Hammen
    Commented Dec 1 at 13:38
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    $\begingroup$ The Vera Rubin Observatory is on Earth, so any KBOs it finds will be visible by an Earth-based telescope. All dwarf planets that have been discovered so far have been discovered by Earth-based telescopes. I make no claim about undiscovered objects! $\endgroup$
    – James K
    Commented Dec 1 at 17:15

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