I suspect a dimension or dimensions we can't observe. But it could just be something we can't grasp. Human meatbags can only see and hear certain wavelengths. The audible spectrum is 20 hertz to 20k hertz. The visible spectrum is about 40--790 terahertz. That leaves a lot of room for things we can't perceive. Just because we can't see or hear something doesn't mean it's not there.