# How do Physicists know that Dark Matter doesn't absorb radiation?

**URL:** <https://boards.straightdope.com/t/how-do-physicists-know-that-dark-matter-doesnt-absorb-radiation/708952>\
**Category:** Factual Questions\
**Created:** [January 4, 2015, 3:52pm UTC](https://boards.straightdope.com/t/how-do-physicists-know-that-dark-matter-doesnt-absorb-radiation/708952 "2015-01-04T15:52:59Z")\
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**Author:** ![septimus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/septimus/32/410_2.png) [@septimus](https://boards.straightdope.com/u/septimus)\
**Post date:** [January 6, 2015, 11:47am UTC](https://boards.straightdope.com/t/how-do-physicists-know-that-dark-matter-doesnt-absorb-radiation/708952/3 "2015-01-06T11:47:00Z")

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> [@Chronos](#):
>
> The laws of thermodynamics absolutely require that if something be able to absorb radiation, it also be able to re-emit it. We can never be absolutely certain, of course, that everything in the Universe actually follows the laws of thermodynamics, but then, the same is true of literally everything else we know.

That’s it? The statement “Dark matter neither emits nor absorbs radiation” is just a phrasing of “Dark matter doesn’t emit radiation; physicists therefore infer from thermodynamic principles that it doesn’t absorb radiation either”?

Is it impossible that some objects have a thermodynamic arrow reversed relative to ours? Would such objects appear as dark matter? If so, the thermodynamic argument becomes circular: We think the thermodynamic arrow points the same way everywhere because radiation absorbers have not been observed. But when we do detect a possible such object we know it’s not a radiation absorber because we think the arrow points the same way everywhere.

I realize this is not a Board where such things can be discussed fruitfully …

> [@Entaglement at the speed of light? or, is "Immediately" a Relative term...](https://boards.straightdope.com/t/entaglement-at-the-speed-of-light-or-is-immediately-a-relative-term/548595/35):
>
> Suppose there was an area of the universe filled tusk to arsehole with invisible giant pink elephants? What would that prove? The universe–our world–is all and only those areas we can observer or infer by observing consistent phenomenon. Nobody outside of an episode of _Dr. Who_ has ever seen a “time-reversed star” or anything of the sort, and we have no evidence by which to infer that such a thing could even exist.

… but I am curious what possibilities for dark matter have been ruled out.

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