All nonphysical dimensions are levels of existence vibrating (for want of a better word) at a higher frequency than the known physical universe. This makes them invisible and undetectable to the normal five senses and to all scientific means available today. Just as different types of energies — light, gravity, x-ray, heat, microwave, etc. — occupy the same space at the same time in the normal physical universe without interfering with each other to any great extent, so do these different dimensional levels coexist in the same space at the same time. Each is of a completely different type, with its own unique properties and spectrum of frequencies.
But it is very difficult to give an easily understandable description that imparts the look and feel of nonphysical dimensions. The astral planes, for example, are part of an incredibly complex nonphysical dimensional level, as compared with the relatively predictable and stable physical dimension. Many of the complexities of the astral planes are virtually unknown. The nonphysical laws governing this dimension (call these astral physics) are fluid and variable in the extreme. On top of this, projectors' minds can greatly affect their perception of the astral dimension. This leaves precious few constant factors for any astral researcher or explorer to work with.
When dealing with the astral dimension you cannot simply point and say, as you can in the physical universe: "This is our planet, Earth; this is its atmosphere and that is outer space, etc". You cannot even point and say, "There is the astral dimension!" Although the astral dimension permeates the entire physical universe, it is, for the greater part, nonspatial. Doorways to the astral exist everywhere and nowhere, both inside and outside the human mind.
That being said, please bear with me as I dissect and expound on the