Cosmic coincidence

In cosmology, the cosmic coincidence is the observation that at the present epoch of the universe's evolution, the energy densities associated with dark matter and dark energy are of the same order of magnitude, leading to their comparable effects on the dynamics of the cosmos. This coincidence is puzzling because these energies have vastly different effects on the universe's expansion—dark matter tends to slow down expansion through gravitational attraction, while dark energy seems to accelerate it. The observed similarity in the magnitudes of these two components' energy densities at this particular epoch in the universe's history raises questions about whether there might be some underlying physical connection or shared origin between dark matter and dark energy. Indeed, some theories attempt to explain this coincidence by proposing that they are different manifestations of the same fundamental force or field.