What Is Dark Matter? The Evidence for the Invisible Mass

Dark matter is matter that we detect only through its gravity. It does not absorb, reflect or give off light, which is why it is called dark (CERN, “Dark matter”^[1]). Our character Dark Matter Host wears that invisibility as a costume, bending light it never shines. This post gathers the evidence, and follows our explainer Why Do Galaxies Spin Weird?.

How much dark matter is there?

NASA describes the universe as made of three parts: ordinary matter about 5%, dark matter about 27% and dark energy about 68% (NASA Science^[2]). CERN puts it another way: dark matter seems to outweigh visible matter roughly six to one (CERN^[1]). The most precise numbers come from the Planck satellite’s map of the cosmic microwave background, which gives a dark matter density Ω_ch² = 0.120 ± 0.001 and an ordinary (baryon) density Ω_bh² = 0.0224 ± 0.0001 (Planck Collaboration, 2020^[3]). The ratio of those two numbers is 0.120 / 0.0224 ≈ 5.4, so about five and a half times more dark matter than ordinary matter.

NASA notes that scientists have been trying to work out what dark matter is for almost 100 years.

Clue 1: clusters of galaxies

In 1937 the astronomer Fritz Zwicky published “On the Masses of Nebulae and of Clusters of Nebulae” in the Astrophysical Journal, weighing clusters of galaxies from how fast their galaxies move (Zwicky, 1937^[4]). CERN summarises the problem that grew from such measurements: galaxies in clusters, like galaxies themselves, move as if held by more mass than we can see (CERN^[1]).

Clue 2: galaxies spin too fast at the edges

For a star in a circular orbit, gravity supplies the pull toward the centre. Setting the two equal gives the orbital speed:

v = √(GM / r)

Here G is the gravitational constant, 6.674 30 × 10⁻¹¹ m³ kg⁻¹ s⁻² (NIST, G^[5]), M is the mass inside the orbit and r is the orbit’s radius.

Worked example

Take an illustrative galaxy with a visible mass of M = 1 × 10⁴¹ kg packed inside r = 3.1 × 10²⁰ m. Then v = √(6.674 × 10⁻¹¹ × 10⁴¹ / 3.1 × 10²⁰) ≈ 1.47 × 10⁵ m/s, about 147 km/s. If there were no more mass further out, a star at twice that radius should move √2 times slower, about 104 km/s. Speeds that stay high instead mean M keeps growing with r, even where the light has faded out.

Vera Rubin and W. Kent Ford measured the rotation of the Andromeda galaxy in 1970 (Rubin & Ford, 1970^[6]), and in 1980 Rubin, Norbert Thonnard and Ford reported the rotational properties of 21 spiral galaxies (Rubin, Thonnard & Ford, 1980^[7]). CERN describes the result: galaxies are rotating with such speed that the gravity of their observable matter could not hold them together (CERN^[1]). That is the story on our Vera Rubin sticker.

Clue 3: the Bullet Cluster

The galaxy cluster 1E 0657-56, nicknamed the Bullet Cluster, formed when two large clusters collided. NASA’s Chandra team describes it as the most energetic event known in the universe since the Big Bang (NASA Chandra, 2006^[8]). The hot gas, which holds most of the ordinary matter, glows in X-rays and was slowed by a drag force, like air resistance, during the crash. Gravitational lensing, the bending of background light by mass, shows that most of the mass sits elsewhere: the dark matter was not slowed, because, as far as we can tell, it interacts very weakly or not at all with itself and the gas except through gravity, so it moved ahead of the gas.

Douglas Clowe and colleagues found an 8-sigma offset between the centre of the total mass and the centre of the ordinary mass. They wrote that this cannot be explained by changing the law of gravity and proves that most of the matter in the system is unseen (Clowe et al., 2006^[9]).

Clue 4: the early universe

The Planck numbers above come from the pattern of hot and cold spots in the oldest light in the sky. In the standard cosmological model that pattern needs both ordinary matter and dark matter in the amounts measured. The same early universe is where the first elements were made, which we cover in How Did the First Elements Form?.

What we do not know: dark matter is an open question

All this evidence is gravitational. The Particle Data Group’s 2024 review states that the fundamental makeup of the large majority of dark matter is, as of yet, unknown (Baudis & Profumo, PDG 2024^[10]). The same review notes that the evidence assumes general relativity is valid. Whether dark matter is a new kind of particle, and what that particle is, remains open. Experiments are searching, and no confirmed detection has been reported in the sources we checked. The Fritz Zwicky sticker and the rest of our dark cast live in the Astrophysics & Cosmology collection.

Quick FAQ

Is dark matter the same as dark energy?

No. Dark matter pulls things together through gravity. Dark energy is the separate name for whatever is driving the universe’s accelerating expansion, about 68% of the total.

Could dark matter be black holes or dim stars?

Planck measures ordinary matter separately, at Ω_bh² = 0.0224, less than a fifth of the dark matter value, so in the standard cosmological model most dark matter is not made of ordinary atoms. Exactly what it is remains unknown.

Has anyone seen dark matter directly?

No. So far it has been detected only through its gravity.

References

1. CERN, “Dark matter”. https://home.cern/science/physics/dark-matter
2. NASA Science, “Dark Matter & Dark Energy”. https://science.nasa.gov/universe/dark-matter-dark-energy/
3. Planck Collaboration, N. Aghanim et al. (2020), “Planck 2018 results. VI. Cosmological parameters,” Astron. Astrophys. 641, A6 (arXiv:1807.06209). https://arxiv.org/abs/1807.06209
4. F. Zwicky (1937), “On the Masses of Nebulae and of Clusters of Nebulae,” Astrophys. J. 86, 217 (DOI 10.1086/143864; metadata verified via Crossref). https://doi.org/10.1086/143864
5. NIST CODATA 2022 recommended values: Newtonian constant of gravitation. https://physics.nist.gov/cgi-bin/cuu/Value?bg
6. V. C. Rubin & W. K. Ford, Jr. (1970), “Rotation of the Andromeda Nebula from a Spectroscopic Survey of Emission Regions,” Astrophys. J. 159, 379 (DOI 10.1086/150317; metadata verified via Crossref). https://doi.org/10.1086/150317
7. V. C. Rubin, N. Thonnard & W. K. Ford, Jr. (1980), “Rotational properties of 21 SC galaxies with a large range of luminosities and radii, from NGC 4605 (R = 4 kpc) to UGC 2885 (R = 122 kpc),” Astrophys. J. 238, 471 (DOI 10.1086/158003; metadata verified via Crossref). https://doi.org/10.1086/158003
8. NASA Chandra X-ray Observatory (2006), “1E 0657-56: NASA Finds Direct Proof of Dark Matter”. https://chandra.harvard.edu/photo/2006/1e0657/
9. D. Clowe, M. Bradač, A. H. Gonzalez, M. Markevitch, S. W. Randall, C. Jones & D. Zaritsky (2006), “A Direct Empirical Proof of the Existence of Dark Matter,” Astrophys. J. 648, L109–L113 (arXiv:astro-ph/0608407; metadata verified via Crossref, DOI 10.1086/508162). https://arxiv.org/abs/astro-ph/0608407
10. L. Baudis & S. Profumo, “Dark Matter,” in S. Navas et al. (Particle Data Group), Phys. Rev. D 110, 030001 (2024). https://pdg.lbl.gov/2024/reviews/rpp2024-rev-dark-matter.pdf

Written by Pixelated Physics. Every factual claim is linked to the source we checked; points that are still debated are labelled open.