They are spirits and can't be seen, although they can play tricks and make you think you can see them. Sometimes it works to their advantage to be discerned, other times they want people to think they don't exist. For example, if they can convince people that they are someone's dead relative, they will reveal themselves. You often hear that after the death of a family member, they make contact with the person. However at this time, most people who have died have no awareness, no life whatsoever. Ec 9:5, 10 says the "dead are conscious of nothing at all." They are in the ground awaiting a future resurrection.
You often hear atheists and skeptics say: "How come demons don't appear to me." Because if they did, the atheist would no longer be an atheist. He might figure: "If wicked spirits exist, so do good ones and maybe God exists."
Demons usually leave people alone if they don't seek them out, in ways like using Ouija boards, etc. But demons and Satan are indirectly influencing everyone. Read 1 John 5:19, which says the "whole world is lying in the power of the wicked one."
