It's an easy topic to dismiss if you've never had dealings with demons. And if you're the type looking for a scientific experiment to proove it, you'll never find one.
My wife played an ouija board when she was 14 years old. At first she thought it was a joke. She asked questions about her future, and they described many details about her future life, including her husband (me). Everything they said was spot on including my height, weight, features, the year we got married, number of children, and many other details.
So, yes. In my opinion they do exist.