What is existing but not noticeable?

Answer 1

bacteria, dust mites, and galaxies are not visually noticeable, but you could argue that their effects are noticeable I suppose.

Speaking from a metaphysical standpoint, you could call this 'the uncanny'. In psychology the uncanny refers to the subconscious, which is rarely fully understood but always in operation.

For example, someone might have a superiority complex that would account for many of their actions, but they wouldn't be able to notice it. However the first objection applies.

objects that have properties of more or fewer dimensions [black holes are a noticeable example that can't in themselves be seen.]

Finally you could argue that anything not noticeable could not possibly have a name, with the exception of the word nothingness, which only has existence as a name and concept but could never have an example.