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:
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 probably some old, faint (massive) white dwarfs with luminosities below $10^{-5} L_{\odot}$, they are not completely invisible and there also seems no reason why their progenitor stars should have been more widely distributed in a halo around the galaxy than the stars we see today.
Microlensing experiments rule out "massive compact halo objects" with $10^{-6}<M/M_\odot<100$, like very cold white dwarfs or black holes, as a significant contributor to dark matter (see for example here).
Most of the dark matter needs to be non baryonic and to interact very weakly with normal matter in order to form the structures in the galaxy clusters and in the microwave background that we see today; 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.
ProfRob
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We created dark matter based on the accepted theory that the universe began with the Big Bang and is still expanding, and that the universe is still too young for black dwarf stars to exist. If the Big Bang theory is wrong, and the universe is immensely older than we theorize, then dark matter could actually be an abundance of black holes or black dwarf stars in the universe.
