The shape of the universe is much more complicated than we expected.

Gravity is always interaction. If there is an object outside the system it pulls material to it. If there is a single gravity center outside the universe that can pull material through one point into it. That pulls the system together because the particle forms energy at low pressure to that point. When something pulls energy away from a certain point that means energy from sides travels to that point.  (Image. Gemini) 

If the particle or gravity center turns into wave movement that thing loses its gravity pothole. That means that the gravity that this particle or gravity center causes is gone. When particles turn into wave movement that erases gravity from the universe. 

If the universe looks like a donut, there can be a cosmic void in the middle of it. That makes it possible. The black holes and the particles turn into the wave movement that we call dark energy. When a particle loses its form as a particle and turns into wave movement it loses its gravity pothole. That means cosmic inflation where the universe's size compared to energy and material expands. That means when the black holes and particles turn into wave movement that loses gravitation from the universe. 

The idea of a doughnut-shaped universe comes from the Big Bang model. The Big Bang sent material away from it. That forms the hole in the material. There is an explanation for the dark energy. In that model, the cold Big Bang before the Big Bang that formed visible material sent that matter away. The dark matter cloud travels before the visible material. The gravitational interaction makes it possible that the visible material cannot fall back to the center of the universe. In that model, the universe looks like a round pancake with a hole in the middle of it. The outside gravity effect drives larger. 

That outside gravity effect doesn't close dark energy away. The dark energy and that hypothetical dark matter cloud may pull the universe larger. Dark energy can form in the events that the black holes go in the low-energy areas or in cosmic voids. That makes those black holes erupt or vaporize. Or if there is a large cosmic void in the universe, that thing makes it possible that the dark energy from in the cases where the particles go in the zero energy area. That area or ultimate cosmic void makes energy flow out from the particles very fast. That turns them into wave movement. That wave movement impacts other waves and it turns its wavelength. 

Image: Gemini

The cosmological models suggest that the universe is always more and more complicated. The space of the universe compared to material and energy grows. And that makes free space in the system. That thing increases entropy and growing entropy turns the universe more and more complicated. 

The geometrical shape of the universe is unknown. There are three possibilities for its geometrical shape. The interesting thing is the so-called Ω (Omega) value. If the Ω>1 gravity wins and the universe collapses in the Big Crunch.  If we look at probability the spherical universe is the only universe's geometrical shape that makes the phoenix universe possible. 

The thing that makes the Phoenix universe interesting is that when the Big Crunch happens. All material falls into a black hole. Then sooner or later that black hole remains alone in the emptiness. And that causes a situation where the material erupts again out from that black hole. That means the new Big Bang.  But the probability for that is only 1/3. Ω>1 only in a spherical universe. 

The universe is more complicated than we realize. It's possible. That the universe has the shape of a doughnut. Or maybe it's a sphere, or a flat pancake, or maybe it's curved. Sometimes is introduced that the shape of the universe is like a wheel, or wheel-shaped pancake there is a hole in the middle of it. The other, not very commonly mentioned form is that the universe is like a wheel. That means the curvature of the universe turns the universe's shape into the wheel. 

"The local geometry of the universe is determined by whether the density parameter Ω is greater than, less than, or equal to 1. From top to bottom: a spherical universe with Ω > 1, a hyperbolic universe with Ω < 1, and a flat universe with Ω = 1. These depictions of two-dimensional surfaces are merely easily visualizable analogs to the 3-dimensional structure of (local) space." (Wikipedia, Shape of the Universe). 

If we think about the universe's shape one of the reasons why we cannot see other universes can be the Event horizon. When energy travels out from the universe in the form of wave movement the lack of resistance can pull that wave movement straight. And that makes it hard to see them. Or if radiation that reaches our universe is far lower energy level than radiation that leaves our universe that means those other universes' radiation cannot reach our universe. 

"Proper distance spacetime diagram of our flat ΛCDM universe. Particle horizon: green, Hubble radius: blue, Event horizon: purple, Light cone: orange." (Wikipedia, Shape of the Universe). 

"Hyperbolic universe with the same radiation and matter density parameters as ours, but without dark energy."(Wikipedia, Shape of the Universe). 

"Closed universe without dark energy and with overcritical matter density, which leads to a Big Crunch. Neither the hyperbolic nor the closed examples have an Event horizon."(Wikipedia, Shape of the Universe). 

If there is an impact wave around the universe. That can deny us to see other universes. Standing waves or impact waves can form only. If there is an outside radiation source. That hypothetical impact wave can form in the cases where outside energy comes from another universe. Or particles that lose their form as particles outside the universe. 

In that case, those particles can turn into wave movement. If there is a plasma shell around the universe and it's energy level is higher than the energy that comes outside the universe. That means the plasma shell can turn outcoming radiation away from the universe. 

In that case, the energy can surround the universe. That means that if the universe has a similar impact wave around it as the solar system that can cause an effect that the impact wave denies the outcoming radiation from traveling into the universe. 

Or maybe it turns into the hole bread that has a so-called thin or weak 3D vertical structure. So the universe looks like a thin donut. The "fat donut" is the scenario where the universe is like a sphere there is a hole. And in some other models, the universe is like a hollow ball. Or maybe it looks like two opposite curved plates that orbit the center of the universe, the place where the Big Bang happened. Or, part of the Big Bang happened. 

If the universe is doughnut shaped that means there is forming an energy ring in the middle of it.  That energy comes from galaxies and stars in its shell. But we must realize that the shape of the universe is unknown. There can be far more structures than one in the universe. In some models, the universe's geometrical shape can be like a galaxy. 

The ball of visible material is surrounded the dark matter. That means dark energy could be the gravitational effect that forms outside the visible universe. This means dark matter pulls the universe larger. The donut-shaped universe explains dark energy as the energy that falls in the middle of the universe and then reflects from there. That can change the wavelength of radiation. 

One of the most interesting questions is the edge of the universe. If the edge of the universe is similar to the edge of the solar system there is a clear standing shockwave. That standing wave or standing plasma wave requires that there is counter radiation. Counter-radiation can form because there are other universes. Or the gravity pothole around objects turns lower. And that makes more objects visible. Without that pothole, energy travels out from particles without resistance. And that destroys material immediately. 

The structure of the universe is not as homogenous as we predict. The gravity centers pull material and quantum fields around them. So that means the material is like in globs around the most massive objects. This causes entropy in the energy waves. Gravity is not the only effect in the universe. Other wave movements and other kinds of effects like energy falling away from the edge of the universe are interesting things. 

https://www.forbes.com/sites/startswithabang/2021/07/21/why-the-universe-probably-isnt-shaped-like-a-donut/

https://www.livescience.com/the-universe-might-be-shaped-like-a-doughnut-not-like-a-pancake-new-research-suggests

https://phys.org/news/2025-01-cosmological-universe-messier-complicated.html

https://en.wikipedia.org/wiki/Shape_of_the_universe

https://en.wikipedia.org/wiki/Ultimate_fate_of_the_universe