What Is Dark Matter?
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Dark matter is a form of matter that appears to make up around a quarter of the universe, exerts gravity, and has never been directly detected. That combination makes it either the biggest gap in modern physics or a sign that we have gravity wrong, and both possibilities are taken seriously.
Why think it exists
The first strong evidence came from the rotation of galaxies. Stars at the outer edge of a spiral galaxy orbit far faster than the visible matter can account for, and at those speeds they should have been flung away long ago.
Measurements by Vera Rubin and colleagues in the 1970s showed this pattern across many galaxies. Either there is a great deal of unseen mass holding them together, or our understanding of gravity at these scales is incomplete.
The other evidence
Galaxy rotation is not the only line of argument, which is what makes the case strong:
- •Gravitational lensing, where light bends around more mass than is visible
- •The cosmic microwave background, whose pattern requires non luminous matter
- •Galaxy cluster dynamics, moving too fast to stay bound without extra mass
- •Structure formation simulations, which only match observations with dark matter included
The bullet cluster
One observation is often described as the strongest single piece of evidence. The bullet cluster is a collision between two galaxy clusters, and it separates the components in a revealing way.
The hot gas, which is most of the ordinary matter, slowed down and piled up in the middle. But gravitational lensing shows the bulk of the mass carried straight through and sits away from the gas. That separation is difficult to explain by modifying gravity, since gravity should track wherever the visible matter is.
What it might be
The leading candidates are particles that interact through gravity and possibly the weak nuclear force, but not electromagnetically, which is why they neither emit nor absorb light.
Weakly interacting massive particles were the favourite for decades, but increasingly sensitive detectors deep underground have repeatedly found nothing, narrowing the possibilities considerably. Attention has shifted towards alternatives including axions and various lighter particles, and searches are ongoing.
The honest position
Dark matter is inferred entirely from its gravitational effects. No experiment has detected a dark matter particle, despite decades of increasingly sensitive searches.
There is a serious minority position, modified Newtonian dynamics and its relatives, arguing that gravity behaves differently at very low accelerations. These models fit galaxy rotation curves well but struggle with clusters and with the cosmic microwave background. Most cosmologists favour dark matter while acknowledging that not finding it is a genuine problem.
Dark matter and dark energy are different
The two are routinely confused and are not related beyond both being poorly understood. Dark matter has gravity and holds structures together. Dark energy appears to push space apart and drives the accelerating expansion of the universe.
The proportions are roughly five percent ordinary matter, twenty seven percent dark matter and sixty eight percent dark energy. Put plainly, everything ever observed directly, including every star, planet and galaxy, accounts for about a twentieth of the contents of the universe, and the rest is named after our ignorance rather than described by our knowledge.
The takeaway
Dark matter is unseen mass inferred from galaxy rotation, gravitational lensing and the structure of the universe, it makes up about a quarter of everything, and despite decades of searching nobody has detected a particle of it.