According to past studies and Pet MD, cats and dogs are capable of seeing far more of the world we live in when compared to humans: “Cats, dogs, and other mammals are thought to see in ultraviolet light, which opens up a whole different world than the one we see, the study explains. UV light is the wave length beyond the visible light from red to violet that humans can see. Humans have a lens that blocks UV from reaching the retina. It was previously thought that most mammals have lenses similar to humans. Scientists studied the lenses of dead mammals, including cats, dogs, monkeys, pandas, hedgehogs, and ferrets. By researching how much light passes through the lens to reach the retina, they concluded that some mammals previously thought not to be able to see UV actually can.”
Anyone who has watched a good show regarding the Bigfoot phenomenon will be familiar with these ideas; Bigfoots have been alleged to see in the same manner as the ones described above, and some people have even argued that they see IR (infrared) light. However, until someone finds a complete specimen (if you’re a Bigfoot believer, that is), both theories must remain just those—although it would help explain why no one has successfully captured a living Bigfoot, since naturally being able to see multiple spectra of light would be a huge advantage in avoiding detection.
Somewhat similarly, some human beings have claimed to see “shadow people” before, although the individuals making the claims also usually experience sleep paralysis. Some witnesses happened to be accompanied by cats, and in these situations the cats seemed to witness the shadow people as well—if terrifying meowing and hissing is any indication of their presence, that is!
In all honesty, it’s quite possible that at least some equivalent of shadow people—“shadow somethings,” if you will—exists. Any good astronomer will tell you that most of the known universe is comprised of largely unknown dark energy and dark matter: a substance that us humans cannot see or interact with, but a substance that top astronomers and physicians are convinced is real, nevertheless.