In discussing string theory, a hypothetical attempt to combine all forces of energy and types of matter into a common theory which imagines the universe to be composed of various types of vibrating strings, Dr. Greene revealed that this theory cannot possibly work unless there are dimensions beyond length, width, height, and time. Most versions of string theory (there are many versions) imagine at least ten dimensions, where the other dimensions are so tiny we can't perceive them.
I've seen this compared to looking at a garden hose from a distance. From a distance, a hose looks looks like it only contains one dimension, length. Up close, of course, it has width and depth as well, and has surfaces contained both on the exterior and interior of the hose. Up close, it's far more complicated than from a distance. Likewise, it is theorized that our real world has multiple dimensions that are invisible because they are very tiny, wrapped up around themselves, too small for humans to perceive even with powerful microscopes.
A subset of string theory that I'm not even going to attempt to explain in any detail here, called "braneworld" or "brane cosmology," imagines the dimensions are not necessarily smaller, but that the entire universe as we know it is trapped within a subset of four dimensions and that all the light and other forces that gives us information about our world are trapped in the same four we inhabit, with the possible exception of gravity. In this theory we would possibly be surrounded by other dimensions on a scale we could see, but the light that gives information to our eyes simply does not enter these other dimensions. This subset of modern physics makes it possible that we are surrounded by an invisible world of measures beyond length, width, height, and time. This made me think of the spiritual world.
Perhaps the world of angels and demons inhabits these other dimensions. Please note that while the "braneworld" idea imagines we would be unable to perceive other dimensions, the reverse would not be true. From the higher dimensions looking down, our world would not at all be invisible. If angels and demons inhabited the other dimensions of braneworld theory, they could be literally right next to us and able to see us, but we would not be able to see them.
Whether true or not, it would be great for a Christian Fiction story to be written from this point of view. Someone should write a tale showing understanding of branes and string theory, showing how the spiritual world surrounding us is compatible with these ideas of what could be real in modern physics, portraying our world existing in full view of these other dimensions invisible to us, prodding skeptics doubting the existence of God to reconsider a belief that's consistent with the latest speculations of Science.
And it just might make a very interesting story setting...