The question of why our universe has a relatively low density compared to hypothetical other universes, if they exist, is still a topic of active scientific research and speculation. Currently, our understanding of the universe's density is based on the observations and measurements of its overall mass-energy content, primarily through studies of cosmic microwave background radiation, galaxy surveys, and other cosmological observations.
The observed low density of our universe, often referred to as the "critical density," suggests that the mass-energy content of the universe is just enough to slow down its expansion but not enough to cause it to collapse under its own gravitational pull. This implies that the universe is spatially flat, where the overall curvature is close to zero.
The reason we are not crushed by the universe's own gravitational pull can be attributed to the interplay between gravitational forces and the expansion of the universe. The universe's expansion acts to counterbalance the gravitational attraction between matter and prevent the collapse of structures on large scales.
The concept of dark energy plays a crucial role in understanding this balance. Dark energy is a hypothetical form of energy that is thought to permeate space and drive the accelerated expansion of the universe. It is believed to counteract the gravitational pull of matter, allowing the universe to expand at an accelerating rate.
The presence of dark energy is inferred from observational data, such as the measurements of the accelerated expansion of the universe based on supernova observations and other cosmological probes. However, the true nature of dark energy remains one of the biggest mysteries in modern physics.
It's worth noting that the specific values of the universe's density and the presence of dark energy are still active areas of research. Scientists continue to investigate various cosmological models and gather observational data to refine our understanding of these fundamental aspects of the universe.