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"Why now?" puzzle

From astronomical observations, it follows that the density associated with dark energy in our universe has the same order of magnitude as the density of the matter content of the universe. That is remarkable - in the past, the matter density will have been much larger than that of the dark energy, and in the far future, the roles will be reversed. Is it coincidence that we make our observations precisely at the time when the two densities are of comparable size, or is there a physical explanation for it?