It really depends on what it is. their are many diffident things that people refer to as demons by the way they look or act but a true demon is something that forms from negative energy produced by living things so yes demons do exist just not here however it is possible for them to come here
Humans used to attribute every single act of nature as being caused by an entity--think sprite, or fairy, or angel--but then science started discovering the real reasons that stuff happens in nature. Most of the supernatural explanations have gone away. We don't think a cherub shoots you with an arrow to make you fall in love, for instance. But evil is still "caused by" demons, or the devil. We are fast learning that, in fact, evil comes from within us, a horrible gift from the evolutionary process. One we can overcome, but still underneath.
I agree with sloza2025 regarding horrible humans that commit demonic acts against innocent ppl or kids but if your referring to non-human demons they only exist if you allow them to...do what you have to do to stay spiritually grounded and don't allow negative thoughts like that to come into your mind..stay focused on the good and positive in life...negative thoughts are like a cancer..they will consume you..don't waste your energy on thoughts of demons..they do NOT exist!