There are so many theories about the true layout and structure of the omni-verse, however they all share the central idea that there is an existence of higher dimensions, or 'realities' beyond that which we can percieve with our limiting five senses. Science has come up with various theories of super-dimensionality, and one of the most revolutionized of these theories to date is known as M-Theory . This 'perspective', as it may also be called, states that our universe (keep in mind that 'universe' encompasses everything that is our physical existence, and 'omni-verse' represents all that exists in every reality) has up to eleven higher dimensions, and that there is a sub-region, or a sort of membrane, between these dimensions that only sub-atomic particles are capable of passing through, and where phenomena such as wormholes manifest. This theory also highlights the possibility of parallel worlds from the idea that this 'sub-atomic sub-region' so to speak can link mirroring planes, or dimensions. Now, given that this is a scientific explanation of the universe supported via complex mathematical evidence, it is merely a model of the structure of our 'physical' world. If one were to imagine what this physical universe might look like, it would show a continuous plane folded up into a very abstract and complex structure, known as a Calabi-Yau shape. This shape may be the structure of our physical universe alone, or it might model another abstract idea representing the existence of parallel worlds; which states that our physical universe is a bubble spawned from a point inside of, or relative to a parallel universe like our own. The spawning of our 'bubble' is said to have been catalyzed by an extremely powerful force eminating from a black-hole eruption. One might even decide to combine these two theories and say that the parallel universe that spawned our own resides on a portion of the calabi-yau plane perpendicular or somewhat relative to our new section of this plane, and that the seperation between these two worlds is the sub-atomic nether region described by M-theory. Going even further into detail, there is a third theory about the structure of the universe stating that all matter is composed of a basic structure known as an atom, which is in turn composed of Quarks, which are again broken down into the smallest and most fundamental substance known as Strings. These 'strings' oscillate at various frequencies which correspond to different states of matter, and thus composing all that we percieve as the physical universe.
But that is where the scientific investigations cease, at the boundaries of our five basic senses. However, each modern cosmological theory provides useful insight for understanding the nature of the higher super-universe. Each sequentially higher dimension, or plane, does not lie at a linear distance from the previous. Instead, they all interpenetrate one another within the same space-time contiuum. Where you are sitting at your computer right now reading this, you are also sitting amidst every other dimensional reality. This is possible because each one has a unique 'frequency' casuing it to exist in a much finer state of matter; finer than air. Much like the way water is able to pentrate soil, and gas able to penetrate both, the finer/lighter sates of matter in the higher planes are able to penetrate all of the denser physical matter. One might say that the strings making up all that is in the omni-verse are vibrating at unique oscillations to create both physical and etheric matter, and even higher vibrational states, just like the freqeuncies of different waves along the elctromagnetic spectrum. As humans, we can only percieve visible light, yet we know of the existence of much higher vibrating waves...And just so our five basic senses are naturally tuned to the frequency of our own realm.
The densest, and lowest frequency world is our familiar Physical Plane. It is three-dimensional just like the reality we naturally percieve. This plane is further divided into four sub-planes: Solid being the lowest and most dense, liquid, gas, and ether. Again, these 'sub-planes' are not literally three dimensional spaces of existence placed linearly on top of one another. As we can see in everyday life, solids, gases, and liquids co-exist with one another everywhere. The fourth 'sub-plane', ether, is somewhat of a 'hyper plane'. This because ether is the first state of matter that exists outside of our natural perception. The state of ether is commonly known as the Etheric Plane, and some systems may identify it as being a seperate reality. In truth, although it requires a heightened perception to acknowledge its existence, ether coexists among the physical. Everything in our world has an etheric double. To visualize what the etheric plane might look like, picture everything around you as being a ghostly object, and appearing to be made of light. This light is brighter than normal light however, and the stars in the sky would even appear much larger and more powerful. When one has their first OBE (out of body experience) floating around one's body in one's room, this will be experienced in the etheric level.
The second level is called the Astral Plane, which encompasses two general sub-planes: The Upper Astral Plane, and The Lower Astral Plane. Everything on this plane corresponds to emotions, and every thought or idea a person has manifests itself on this plane before taking form in the physical. Physic phenomena such as clairvoyanceand remote viewing also occur here because of the ability to percieve thought waves as an energy force on this plane. When a person feels a very powerful emotion, and devotes most of their mental energy on one thought, the energy field that manifests a form in the Astral Plane can become powerful enough to sustain itself temporarily. This new entity, known as a thought-form, can even gain a 'personality' via the energy drawn from the emotion(s) delivered from its creator. If concentrated on for a long enough time, and supplied with enough energy, a poltergeist can be created which usually thrives off of negative energy and is capable of causing damage. The Astral Plane visually is hard to describe, as the environment is usually created by the travelers thoughts; but physically, this plane is liquid in nature, much like the characteristics of plasma, and everything you percieve here is abnormally vibrant as if your were seeing in HD, or on an LSD trip. At night, the dreamworlds that one experiences typically takes place in the Lower Astral Plane. When conciouslly travelling to this world, especially as a begginer in Astral Projection, one might find that they are sort of magnetised to the Lower Astral Plane. This region is typically a dark and mysterious environment, usually representing negative emotions, and one may encounter sneaky and impish appearing entities. On the other hand, The Upper Astral Plane is more like a renassaince of beauty, color and happy emotions and energies. Also, the Astral Plane is what Catholicism or other religions refer to as purgatory, because this is the plane where souls come who are awaiting incarnation. The Lower Astral Plane is where impure souls must experience hardships that lead to spiritual guidance and cleansing before contuing on their journey to elysium.
The third level is the Buddhic, or Intuitional Plane. This Plane is divided into the (higher) Mental region (see article, Mental Plane) and (lower) Causal region (see article, Causal Plane). The Mental region is the highest plane reachable by an astral traveller, and it provides the energy we tap into when critically thinking or using intuition, etc. The far edges of the Mental Plane are foggy and disorienting because it represents the area of the mind where 'physical' or undivine perception can not percieve any farther; this nether region marks the boundary between the Atmic Plane. The Causal region is where space-time manifests. This is where all our memories reside; by memory referring to the energy imprint of an experience - past, present, or future. This is also the level where one can access their Akashic Records, or energetic records of all experiences from the previous and past lives; also known as one's Karmic Cycle.
The fourth level, the Atmic Plane, or commonly referred to as the Soul Plane or Elysium; is the beggining of the higher super dimensional worlds. This level, along with the following, can only be perceived directly by the soul. There are accounts however, of travellers reaching a high enough vibration to meet a spiritual guide, or even a master, who helped the traveller through the nether region of the Mental Plane into The Atmic. There, they witnessed a powerfully serene and peaceful environment, where the sound of a flute is said to be heard, or vibrationally felt coursing through the body.
The Fifth and sequentially higher planes are reffered to as Nirvanna, ParaNirvana, MahaParaNirvana, Monadic, Logoic, or Divine, in both sanskrit and english terms. Although little is known to humans about these planes, they are said to be divided into three general regions; The Realm of Life Spirits, Realm of Virgin Spirits or Devas (angels) or Saints, and the Realm of GOD.
Golden Wisdom Temples - On each unique plane of existence, there resides a temple of wisdom, where souls and travellers can go to gain spiritual knowledge and guidance on just about any subject from the Masters or Teachers. Just as on Earth we had the teachers of Jesus, Buddha, Lao-Tzu, Mohammad, etc who learned from masters of a higher plane, each significant plane has its own teacher(s) who are essentially souls that have been purified and gained spiritual knowledge but have chosen to stay on a lower plane to guide newer souls along their spiritual journeys.