dark matter

Weigh a spiral galaxy by the light of its stars, then watch how fast its outer stars orbit, and the numbers do not add up: the stars in the outskirts circle far too quickly to be held by the visible mass alone; they should have flung off long ago. Something unseen is gripping them. That something, five times more abundant than all the ordinary matter in stars, gas, and planets combined, is dark matter. It answers the question of what holds galaxies and clusters together, and why the cosmos grew the structures it did.

Dark matter is matter that gravitates but neither emits nor absorbs light, detectable so far only through its gravity. The evidence is overlapping and strong: flat galaxy rotation curves (orbital speed staying high at large radius instead of falling as Kepler's 1/sqrt(r) would demand), gravitational lensing that bends light by more mass than we see, the pattern of CMB ripples, and the way galaxies clustered over cosmic time. It makes up about 27 percent of the total energy content of the universe, against roughly 5 percent for ordinary (baryonic) matter. It is thought to be cold, meaning slow-moving when structure formed, and non-baryonic, not made of protons and neutrons, since Big Bang nucleosynthesis and the CMB both limit how much ordinary matter can exist.

The honest state of the field: we know dark matter is there from its gravity, but we do not know what it is. Leading candidates are new particles, such as weakly interacting massive particles or axions, but decades of direct-detection experiments have found nothing conclusive. A minority view holds that no new matter exists and that gravity itself must be modified on galactic scales; the collision of galaxy clusters, where the lensing mass separates cleanly from the visible gas, is the strongest evidence against that alternative.