coincidence problem

The "coincidence problem" in astronomy refers to the observation that the universe's expansion rate and the density of dark matter are currently very similar, despite evolving differently over cosmic history. This near-equality, particularly the dominance of dark energy in recent cosmic times, is puzzling because dark energy must have been negligible in the early universe for structures like galaxies to form.1

This fine-tuning of cosmic parameters, allowing for the existence of life, is also related to the anthropic principle, which suggests that the universe's properties must be compatible with observers. The precise values of fundamental constants and the universe's expansion rate appear to be remarkably coincidental, enabling the formation of elements necessary for life and the development of complex structures. Understanding why these values are so finely tuned remains a significant challenge in modern physics.21