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Today we understand that a black dwarf star represents a hypothetical star that is the result of the complete consumption of the energy of a white dwarf which is the remnant of a star of little or half mass (1 solar mass), once all its hydrogen has been consumed or expelled. This rest is a dense piece of "degenerated matter" that slowly cools and crystallizes by emission of heat radiation. So, if these objects (not yet observed) do not emit light but interact gravitationally with the surrounding matter, we can not say that dark matter may be black dwarf stars that are contained within the galactic halo? To discard weak interaction particles such as WIMP's or its opposite, the MACHO's ("massive compact halo objects")

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Three reasons:

  1. As you correctly point out, black dwarfs are "hypothetical objects". There has been insufficient time since the first stars were born for white dwarfs to cool below about 3000 K. i.e. Whilst there are faint white dwarfs with luminosities below a few $10^{-5} L_{\odot}$, they are not invisible.

  2. Microlensing experiments rule out "massive compact halo objects", like cold white dwarfs or black holes as a significant contributor to dark matter.

  3. Most of the dark matter needs to be non baryonic and to interact very weakly with normal matter in order to form the structures that we see today in the universe; and to reconcile the inferred primordial abundances of helium, deuterium and lithium with the total amount of matter deduced to be in galaxies and clusters of galaxies. Cold white dwarfs are baryonic, so cannot represent the bulk of dark matter.

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    $\begingroup$ Most of the dark matter needs to be non baryonic in the Big Bang model since the galaxies wouldn't have formed. If one needs to invoke "magic" to get a cosmological model to form galaxies, then model is probably broken. $\endgroup$ – Cinaed Simson Apr 8 '19 at 3:04
  • $\begingroup$ @CinaedSimson Indeed the model is/was broken. GR w/out dark matter cannot explain structure formation. Feel free to add your answer. $\endgroup$ – Rob Jeffries Apr 8 '19 at 7:06
  • $\begingroup$ @Rob_Jeffries: but when you change the data instead of the theory, isn't that the definition of junk science? $\endgroup$ – Cinaed Simson Apr 13 '19 at 23:30
  • $\begingroup$ As I said, feel free to add your answer, if you have one. @CinaedSimson $\endgroup$ – Rob Jeffries Apr 13 '19 at 23:35
  • $\begingroup$ And the big bang model is not a "theory". $\endgroup$ – Rob Jeffries Apr 13 '19 at 23:44

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