coincidence problem
Some puzzles are not about a wrong number but about suspicious timing. Throughout cosmic history, the density of matter and the density of dark energy have changed very differently. Matter thins out as the universe expands — spread the same stuff over more space and it gets more dilute. Dark energy, if it is a cosmological constant, does not thin out at all; its density stays fixed. So matter dominated long ago, dark energy will dominate forever after, and they are roughly equal only during one brief cosmic moment. The coincidence problem asks: why do we happen to live in exactly that moment?
Consider the timeline. In the early universe, matter was vastly denser than dark energy — by enormous factors. In the far future, as space keeps expanding, matter will become utterly negligible compared with the unchanging dark energy. The crossover, when the two are comparable, lasts only a small slice of the universe's history. Yet right now, today, the matter density and the dark energy density are within a factor of a few of each other (about 31% versus 69%). Out of all the eras we could have found ourselves in, we are sitting almost exactly at the handover.
Is this a genuine mystery or just an accident? Opinions differ, and this is openly debated. One response is that it is simply a coincidence and we should not overinterpret it. Another is that it points to new physics — perhaps a dynamical dark energy like quintessence that is somehow linked to matter and naturally tracks it, easing the fine timing. A third, more philosophical response invokes selection: galaxies, stars, and observers can only form during a particular epoch, so perhaps it is no accident that thinking beings arise around the crossover. The coincidence problem is a softer puzzle than the cosmological-constant problem — some physicists doubt it is a real problem at all — but it keeps tugging at our sense that the timing is too neat.
Run the cosmic clock forward and back: a billion years after the big bang, matter outweighed dark energy thousands of times over; a hundred billion years from now, dark energy will outweigh matter by similar factors. Only in the present epoch are they close (roughly 31% matter to 69% dark energy) — and that is when we exist to notice.
Matter and dark energy are comparable only briefly — and we happen to live during that window.
Unlike the cosmological-constant problem (a hard quantitative mismatch), the coincidence problem is more interpretive — some physicists consider it a genuine clue, others a non-problem dressed up by selection effects. Be wary of treating it as settled either way.