# Could dark matter be...heat? **URL:** \ **Category:** Factual Questions\ **Created:** [January 8, 2026, 7:58pm UTC](https://boards.straightdope.com/t/could-dark-matter-be-heat/1026480 "2026-01-08T19:58:29Z")\ **Posts on this page:** 14\ **Page:** 1
**Author:** ![EnolaStraight](https://avatars.discourse-cdn.com/v4/letter/e/41988e/32.png) [@EnolaStraight](https://boards.straightdope.com/u/EnolaStraight)\ **Post date:** [January 8, 2026, 7:58pm UTC](https://boards.straightdope.com/t/could-dark-matter-be-heat/1026480/1 "2026-01-08T19:58:29Z")
The Galaxy rotates as if it contains more mass than can be seen. Due to E=mc^2, matter and energy are interchangeable. Heat is a form of energy. Energy can distort spacetime like mass. Can a galaxy full of heat from stars and black holes and whatnot account for “missing mass”? Furthermore, could the cold, massless voids between galactic superclusters be Dark Energy? ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [January 8, 2026, 8:03pm UTC](https://boards.straightdope.com/t/could-dark-matter-be-heat/1026480/2 "2026-01-08T20:03:16Z")
I do not think so. As space expands the energy density in the universe from heat would decrease (more space, same amount of heat). Dark energy’s pressure (for lack of a better word) seems to remain constant as space expands. ---
**Author:** ![Der\_Trihs](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/der_trihs/32/233_2.png) [@Der\_Trihs](https://boards.straightdope.com/u/Der_Trihs)\ **Post date:** [January 8, 2026, 8:06pm UTC](https://boards.straightdope.com/t/could-dark-matter-be-heat/1026480/3 "2026-01-08T20:06:42Z")
> [@EnolaStraight](#): > > Due to E=mc^2, matter and energy are interchangeable. > > Heat is a form of energy. > Energy can distort spacetime like mass. > > Can a galaxy full of heat from stars and black holes and whatnot account for “missing mass”? Nope. The indications are that the _great majority_ of matter in the universe is dark matter; if there was that much heat we’d know because the galaxy in question would be literally exploding like a supernova. ---
**Author:** ![xtenkfarpl](https://avatars.discourse-cdn.com/v4/letter/x/c2a13f/32.png) [@xtenkfarpl](https://boards.straightdope.com/u/xtenkfarpl)\ **Post date:** [January 8, 2026, 10:42pm UTC](https://boards.straightdope.com/t/could-dark-matter-be-heat/1026480/4 "2026-01-08T22:42:18Z")
> [@EnolaStraight](#): > > Heat is a form of energy. > Energy can distort spacetime like mass. A standard example of a mistake in syllogistic reasoning is: All pennies are made of copper. This thing is made of copper. Therefore it is a penny. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [January 8, 2026, 11:10pm UTC](https://boards.straightdope.com/t/could-dark-matter-be-heat/1026480/5 "2026-01-08T23:10:01Z")
I mean, heat does have mass. But unless you’re at a truly ludicrous temperature, it’s a very, very small amount of mass. ---
**Author:** ![Der\_Trihs](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/der_trihs/32/233_2.png) [@Der\_Trihs](https://boards.straightdope.com/u/Der_Trihs)\ **Post date:** [January 8, 2026, 11:14pm UTC](https://boards.straightdope.com/t/could-dark-matter-be-heat/1026480/6 "2026-01-08T23:14:21Z")
Yes. There’s even the concept of a [kugelblitz](https://en.wikipedia.org/wiki/Kugelblitz_(astrophysics)), a black hole created entirely from energy. > A **kugelblitz** (German: [[ˈkuːɡl̩ˌblɪt͡s]](https://en.wikipedia.org/wiki/Help:IPA/Standard_German) [ⓘ](https://en.wikipedia.org/wiki/File:De-Kugelblitz.ogg)) is a [theoretical](https://en.wikipedia.org/wiki/Theoretical_physics) astrophysical object predicted by [general relativity](https://en.wikipedia.org/wiki/General_relativity). It is a concentration of [heat](https://en.wikipedia.org/wiki/Heat), [light](https://en.wikipedia.org/wiki/Light), or [radiation](https://en.wikipedia.org/wiki/Radiation) so intense that its energy forms an [event horizon](https://en.wikipedia.org/wiki/Event_horizon) and becomes self-trapped. In other words, if enough [radiation](https://en.wikipedia.org/wiki/Radiation) is aimed into a region of space, the concentration of energy can warp [spacetime](https://en.wikipedia.org/wiki/Spacetime) so much that it creates a [black hole](https://en.wikipedia.org/wiki/Black_hole). But that doesn’t mean you can significantly curve space without so much energy that it would be _very_ noticeable. And, well, it’s called “dark matter” in the first place because we _can’t_ see it. ---
**Author:** ![FlikTheBlue](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fliktheblue/32/10972_2.png) [@FlikTheBlue](https://boards.straightdope.com/u/FlikTheBlue)\ **Post date:** [January 8, 2026, 11:37pm UTC](https://boards.straightdope.com/t/could-dark-matter-be-heat/1026480/7 "2026-01-08T23:37:36Z")
> [@Der\_Trihs](#): > > But that doesn’t mean you can significantly curve space without so much energy that it would be _very_ noticeable. And, well, it’s called “dark matter” in the first place because we _can’t_ see it. Wouldn’t that much energy in one spot just get converted into matter anyways, like when energy was converted into protons and electrons during the Big Bang? ---
**Author:** ![Der\_Trihs](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/der_trihs/32/233_2.png) [@Der\_Trihs](https://boards.straightdope.com/u/Der_Trihs)\ **Post date:** [January 9, 2026, 2:09am UTC](https://boards.straightdope.com/t/could-dark-matter-be-heat/1026480/8 "2026-01-09T02:09:34Z")
> [@FlikTheBlue](#): > > Wouldn’t that much energy in one spot just get converted into matter anyways, like when energy was converted into protons and electrons during the Big Bang? I don’t…_think_ so. It’s a lot of energy, but a galaxy is really huge so it would be spread out quite a lot. But that’s a question well beyond my knowledge of physics to be sure of. ---
**Author:** ![phs3](https://avatars.discourse-cdn.com/v4/letter/p/8491ac/32.png) [@phs3](https://boards.straightdope.com/u/phs3)\ **Post date:** [January 9, 2026, 6:42pm UTC](https://boards.straightdope.com/t/could-dark-matter-be-heat/1026480/9 "2026-01-09T18:42:47Z")
Hah, excellent example, but it won’t age well! Arguably it already hasn’t, since pennies haven’t actually been even mostly copper since 1982, and haven’t been 100% copper since 1857. Yes, I’m being pedantic, but with a smile on my face–so typical for something “everybody knows” to become no longer true, such that at some point it will just seem wrong! And you kids get off of my lawn. ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [January 9, 2026, 7:36pm UTC](https://boards.straightdope.com/t/could-dark-matter-be-heat/1026480/10 "2026-01-09T19:36:15Z")
As I understand it, dark matter does not interact with ordinary matter; it only makes its presence known through gravity. “Heat” is photons and photons interact via electromagnetic forces. Current theory might be entirely wrong, but that’s very low probability. Also, there are no “massless” voids, merely voids with a low number of galaxies, plus whatever dust and gas and rogue stars that are too faint to easily see with our current technology. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [January 10, 2026, 1:47am UTC](https://boards.straightdope.com/t/could-dark-matter-be-heat/1026480/11 "2026-01-10T01:47:18Z")
> [@Der\_Trihs](#): > > It’s a lot of energy, but a galaxy is really huge so it would be spread out quite a lot. It really can’t be, though. Heat is the kinetic energy of a bunch of particles going in a bunch of different directions. Which means that you also have the mass of those particles. And unless the particles are moving at relativistic speeds, the heat will be much less than the rest mass. ---
**Author:** ![Ancient\_Nerd](https://avatars.discourse-cdn.com/v4/letter/a/ed655f/32.png) [@Ancient\_Nerd](https://boards.straightdope.com/u/Ancient_Nerd)\ **Post date:** [January 11, 2026, 7:43am UTC](https://boards.straightdope.com/t/could-dark-matter-be-heat/1026480/12 "2026-01-11T07:43:05Z")
> [@Chronos](#): > > unless the particles are moving at relativistic speeds, the heat will be much less than the rest mass. Temperature is a function of how fast those particles are moving. If they were moving at relativistic speeds, the matter would be really, really hot. ---
**Author:** ![MikeS](https://avatars.discourse-cdn.com/v4/letter/m/919ad9/32.png) [@MikeS](https://boards.straightdope.com/u/MikeS)\ **Post date:** [January 11, 2026, 2:07pm UTC](https://boards.straightdope.com/t/could-dark-matter-be-heat/1026480/13 "2026-01-11T14:07:35Z")
The problem is that if they were moving at relativistic speeds then the galaxy wouldn’t last long, because they would have enough energy to fly away from each other and dissipate. Since [checks notes] the galaxy hasn’t yet flown apart, we can conclude that its constituents aren’t traveling at the speed of light. Physicists do think about unknown relativistic particles as a possible constituent of dark matter; it’s known as [hot dark matter](https://en.wikipedia.org/wiki/Hot_dark_matter), to distinguish it from particles that are moving at much less than the speed of light (which are [cold dark matter](https://en.wikipedia.org/wiki/Cold_dark_matter).) But having a certain amount of cold dark matter is required to get galaxies to form in the early Universe; hot dark matter is (roughly speaking) moving too fast to clump together and attract regular matter to form galaxies. So you still need som unknown slow-moving particles to explain things. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [January 11, 2026, 5:09pm UTC](https://boards.straightdope.com/t/could-dark-matter-be-heat/1026480/14 "2026-01-11T17:09:25Z")
> [@MikeS](#): > > Since [checks notes] the galaxy hasn’t yet flown apart As an aside, I’m quite fond of scientific observations based on facts like “The galaxy hasn’t flown apart yet”, or “Mars is still in its orbit”, or the like. You can reach solid conclusions that way surprisingly often.