The question of why our universe appears to have a relatively low density compared to other hypothetical universes, if they exist, is speculative and remains unanswered within the current scientific understanding. It is important to note that the density of our universe is not inherently low; it is the result of the observed balance between matter, energy, and the expansion of space.
The observed density of the universe plays a crucial role in its overall dynamics. If the density were too high, the gravitational forces would cause the universe to collapse in on itself. On the other hand, if the density were too low, the universe would expand at an accelerated rate without sufficient gravitational pull to slow down the expansion.
The current understanding, supported by observations and the prevailing cosmological model known as the Lambda-CDM model, suggests that our universe has a critical density. This critical density is related to the balance between the matter (including both visible and dark matter) and energy content of the universe, as well as the effects of dark energy, which is thought to be responsible for the observed accelerated expansion.
The expansion of the universe, driven by dark energy, counteracts the gravitational attraction between matter and prevents the universe from collapsing under its own gravitational pull. The specific value of the critical density and the interplay between matter, energy, and dark energy determine the overall fate and geometry of the universe.
As for the existence of other universes, if they exist in the context of theories like the multiverse hypothesis, it is speculative and remains an area of active research. Different regions of a hypothetical multiverse might have different properties, including variations in densities. However, the exact mechanisms and properties of such universes, and why our universe appears to have a specific density, are currently not well understood and remain subjects of ongoing scientific investigation and theoretical exploration.