Dark Matter Host: Gravitational Lensing / Invisible Mass
Cosmology & the Sky arc, page 139 of 193. Back to The Pixelated Physics Universe
Who they are
Meet Dark Matter Host. Most of the mass holding galaxies together has never been seen. Dark matter gives off no light we can detect, yet its gravity shapes galaxies and bends light. In the Bullet Cluster, two colliding galaxy clusters, the hot gas slammed together and stalled, but lensing shows most of the mass sailed on with the galaxies: mass where the gas isn't. About five-sixths of all matter is something never directly detected. Dark Matter Host wears it in pixel art on a unisex tee.
Dark Matter Host is a character of the Cosmology & the Sky arc: drawn in pixels, ruled by one piece of real physics, and never allowed to break it.
The physics
When two galaxy clusters collided in the Bullet Cluster, their hot gas slammed together and stalled, but bent background light shows most of the mass sailed on with the galaxies. Orbital speeds and lensing both find far more mass than stars and gas can account for: about five-sixths of all matter is something never directly detected.
$$ \begin{gathered} v(r)^{2} = \frac{G M(r)}{r} \;\Rightarrow\; M(r) = \frac{v^{2} r}{G} \\ \theta_E = \sqrt{\frac{4GM}{c^{2}}\,\frac{D_{LS}}{D_L D_S}} \end{gathered} $$
| Symbol | Meaning | SI unit |
|---|---|---|
| $v(r)$ | orbital speed at radius r | m/s |
| $M(r)$ | mass enclosed within r | kg |
| $r$ | galactocentric radius | m |
| $\theta_E$ | Einstein radius | rad |
| $D_L, D_S, D_{LS}$ | angular-diameter distances to lens, to source, lens→source | m |
Status: part of this topic is unsolved, unconfirmed or debated. The text above says which part; treat it as an open question, not settled fact.
Sources
- Rubin & Ford (1970) Rotation of the Andromeda Nebula, ApJ 159, 379
- Zwicky (1937) On the Masses of Nebulae and of Clusters of Nebulae, ApJ 86, 217
- Encyclopaedia Britannica — Dark matter
- PDG 2024 review — Dark matter
- NASA Science — Dark Matter
- Planck Collaboration (2020) Planck 2018 results VI. Cosmological parameters, A&A 641, A6
- Clowe et al. (2006) A Direct Empirical Proof of the Existence of Dark Matter, ApJ 648, L109
Crossovers
Dark Matter Host meets Cavenda (Weighing the World 1798). Cavenda measured gravity between lead spheres in a room. Dark Matter Host is seen only through gravity, from galaxies and lensing. Both lean on Newton's constant G, the weakest coupling on the bench.
Dark Matter Host meets Void Whisperer (Dark Energy Expansion). Void Whisperer and Dark Matter Host make up most of the cosmic budget, and neither has been seen directly. One pulls matter together; the other pushes the expansion faster. What each is made of remains unknown.
Wear the lore
- Dark Matter Host – Gravitational Lensing / Invisible Mass Physics T-Shirt
- Related infographic poster: The Cosmic Budget – Plate 074 Physics Poster
Concept keywords: Gravitational Lensing / Invisible Mass, Dark Matter Host, Cosmology & the Sky, physics lore, Pixelated Physics