I have never seen one but I have not dismissed the possibility that they do exist.
We might not be able to perceive it with out senses and they aren't physical manifestations but that doesn't mean that things such as "Intent" "Grudge" or "hatred" can't leave a perceived reaction.
Some people's senses are more highly attuned to something, not saying it's true or not but possible.
Also, do some research on DMT, the spirit molecue and maybe you'll find yourself opening up to different possibilities.