There is the possibility that Dark Energy will turn weaker, and the universe will end in a Big Crunch.

"An illustration of the Dark Energy Spectroscopic Instrument (DESI) year-one data, showing a slice of the larger 3D map that DESI is constructing during its five-year survey. (Image credit: DESI Collaboration/KPNO/NOIRLab/NSF/AURA/P. Horálek/R. Proctor)" (Space.com, Dark energy could be getting weaker, suggesting the universe will end in a 'Big Crunch')

Dark energy is the thing that dominates the universe. That energy rips particles and the universe into pieces. And then the ultimate end of the universe should be the "Big Silence" that is better known as Big Freeze. The end of the universe is called the Ultimate fate. 

But then, we can think that the source of dark energy is in some kind of interaction between wave movement and superstrings that are thin wave movement. When larger-sized waves impact superstrings, that energy causes them to oscillate. And that could be the source of the dark energy.

And if the distance between those superstrings turns long enough. That thing turns that interaction weaker. The mysterious weakly interacting massive particles (WIMP), or dark matter particles, can be standing waves between those strings. And in that model, they could be virtual particles that reflect wave movement. 

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 Ω = 1, the universe is flat.

If Ω > 1, there is positive curvature.

If Ω < 1, there is negative curvature.

Only if  Ω>1 the ultimate fate of the universe should be a Big Crunch. 

(Wikipedia, Shape of the universe)

The thing that determines the ultimate fate of the universe is the geometric shape of the universe. The spherical universe can fall into the Big Crunch. Two other possible shapes, hyperbolic and flat universes, will expand forever. But then we must realize one thing. Researchers believe that the universe is flat. The fact is that the material that we can see is only 5% of the universe. The rest of the universe is invisible gravity interaction, known as dark matter (26%) and dark energy (69%). 

But if we look at the form of explosion in the universe, they are all some kind of balls. And if the dark matter is released in some kind of explosion, that means the shape of the universe should be a ball. Or maybe the universe has no pure shape at all? 

What if the universe looks like a rugby ball? But then we can rethink the universe as an entirety. There are many theories about dark universes. And even universes lurking in black holes. 

Theories about the form of the universe create a model that maybe the universe has a ball- or rugby-ball-shaped structure. And it's possible. That the universe involves some kind of internal structures that are invisible to us. 

In some models, there is a flat layer in the middle of the universe. That means the universe looks like a galaxy. The dark matter could be like a halo around the flat universe that is like a circle. Or in some other model, the visible universe where we live is the ball, and there is a flat universe in the universe's equator. 

The universe's equator forms when the material that the Big Bang released forms a spherical form. There is a theory. In the middle of the universe is a mass centrum, the extremely high-mass black hole, and the rest of the material orbits that black hole. 

But the thing with dark energy is that maybe dark energy forms when energy that the visible universe forms hits the superstrings or plasma wave that travels ahead of the universe. That thing means that the radiation reflects back from the hypothetical shockwave, that the Big Bang sent. All stars have that kind of impact wave around them. That thing can make the maser-emission that looks like vacuum bombs. 

In some other models, the light that comes straight in the line with black holes can travel a little bit slower than photons that have no black hole at the back of their line. That causes impacts. And that raises those photon's energy level higher than it should. 

https://chandra.harvard.edu/xray_astro/dark_energy/

https://science.nasa.gov/universe/the-universe-is-expanding-faster-these-days-and-dark-energy-is-responsible-so-what-is-dark-energy/

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

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

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

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

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

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

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

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

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

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

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