The current understanding based on observational evidence is that the expansion rate of the universe is actually accelerating, not slowing down. This discovery was made in the late 1990s and was awarded the 2011 Nobel Prize in Physics. The acceleration of the expansion is attributed to a mysterious force called dark energy, which appears to dominate the energy content of the universe.
While the expansion is accelerating, it is important to note that this does not imply that the universe will contract back to a single point or result in another Big Bang. The concept of a "Big Crunch," where the universe collapses under its own gravity, was a possible scenario in earlier cosmological models. However, current observations and measurements indicate that dark energy is dominant and will continue to drive the expansion, potentially even accelerating it further.
Based on our current understanding, it is hypothesized that the universe will continue to expand indefinitely, with galaxies and other cosmic structures becoming increasingly isolated from one another over vast timescales. This scenario is often referred to as the "heat death" or "Big Freeze" of the universe. In this scenario, the universe will become progressively colder and sparser as it expands, eventually leading to a state of maximum entropy where no energy or matter is available for further processes.
It's important to remember that our understanding of the universe and its ultimate fate continues to evolve as scientists conduct more research and gather new data. The nature of dark energy and the long-term fate of the universe remain active areas of study and are subjects of ongoing scientific investigation.