If dark energy did not exist, the fate of the universe would depend on the balance between the gravitational forces pulling matter and energy together and the expansion of the universe due to the initial Big Bang. The presence or absence of dark energy plays a significant role in this balance.
Dark energy is a theoretical form of energy that is postulated to exist to explain the observed accelerated expansion of the universe. It is often associated with the cosmological constant, which was introduced by Albert Einstein in his theory of general relativity. Dark energy is thought to contribute to a repulsive gravitational effect that counteracts the attractive gravitational pull of matter.
If dark energy were absent, and there were no other unknown factors or mechanisms at play, the universe's expansion might slow down over time due to the gravitational attraction between matter and energy. However, it does not necessarily mean that the universe would inevitably collapse in on itself.
The ultimate fate of the universe depends on various factors, including its overall density, the amount and distribution of matter and energy, and the geometry of space. If the universe had a high enough density, it could lead to a "Big Crunch" scenario, where the expansion reverses, and the universe collapses in on itself.
On the other hand, if the universe had a low enough density, it could continue expanding forever, eventually becoming very cold and dilute. This scenario is sometimes referred to as the "Big Freeze" or "Heat Death" of the universe, where all usable energy is spread out and unavailable for further processes.
It's important to note that our understanding of these concepts is based on current theories and observational data, which are subject to change as scientific understanding advances. The precise nature of dark energy and its effects on the fate of the universe are still active areas of research in cosmology.