I’m a little surprised no one has mentioned rods yet, even though they don’t actually exist.
Your next best bet for an invisible lifeform would either be weakly interacting particles or dark matter.
Of course, weakly interacting particles are [drum roll] weakly interacting and therefore not likely to form a living being.
Dark matter is so named because we honestly don’t know what it is. Dark matter even accounts for a larger percentage of the universe by weight than normal matter. From a science fiction standpoint, invisible dark-matter creatures are a gold mine because there’s just enough that we don’t know to fill in some details. From a more serious scientific view, probably not.
Have you read anything on the cat parasite taxoplasmosis?I am not sure if I have the name right.Seems like it is the closest organism to a alien that I’ve heard of.Postulating out,what else is out there that influences our behavior?We know so little.
Yes, it’s an interesting critter, but it’s not invisible, which is what the OP was asking about.
Or another way to say “clear.” Behold a substance called “glass.” And another called “water.” We don’t reflect radio waves at all really, but that doesn’t mean we absorb them all. They pass right through us.
Yes, glass and water are transparent. But glass and water aren’t invisible. You can see glass objects, even though you can also see through them. You can see a blob of water, even though you can see through water. This is because glass and water and plexiglass and various other clear substances refract light at a different rate than air.
The first thing I thought of when I read the op was the ultraviolet coloring in flowers.
The coloring aspect doesn’t meet the OP’s strict condition of total invisibility to humans but it does point out that there are visual cues for other organisms (insects) that support the decorated organisms welfare but are unobservable to the human eye.
Right, my point was there are more than two options when light strikes a surface; in addition to being reflected or absorbed, it can also pass through the object (usually with a change in velocity, causing refraction). But if we put the glass into a substance with the same refractive index (like dipping a glass pipette into a vial of immersion oil) the glass will disappear.
The eyes of the mantis shrimp have cells that can detect circularly polarized light. Circularly polarized light is almost nonexistent in nature. One of the few places that it is found is in light reflected from certain patches on the carapace of the mantis shrimp. Mantis shrimp are social creatures, and it is suspected that they use this to identify and signal to each other.
It’s not quite what the OP asked, but it is a creature which uses a type of light found essentially nowhere else as a secret communication method.
Not all mantis shrimp have this capability, but some do. I believe it aklso helps identify some predators, since they, too , reflect circularly polarized light.
Mantis shrimp can mostly detect polarized light. So can other creatures, including horseshoe crabs and honeybees. It’s believed they use it for navigation. Humans can detect polarized light, too, but to a much lesser degree.
If you’re looking for differing abilities, mamny insects see further into the ultraviolet than people do. And pit vipers have what amounts to a very blurry set of infrared "eyes’ in their pits.