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According to the $\Lambda$CDM parametrization of the Standard Model of Big Bang cosmology, the universe contains a cosmological constant $\Lambda$ associated with $73\%$ dark energy, $23\%$cold dark matter (CDM) and $5\%$ ordinary matter.
I have also read from Kolb and Turner's book that the Cold dark matter leads to a "bottom-up" formation of structure in the universe while hot dark matter would result in a "top-down" formation scenario; since the late 1990s, the latter has been ruled out by observations of high-redshift galaxies such as the Hubble Ultra-Deep Field.
$\bullet$ How do we know that the dark matter is cold or nonrelativistic?
$\bullet$ What is the percentage of hot dark matter in $\Lambda$CDM parameterization? Is the possibility of dark matter totally ruled out?
Addendum Existence of CDM does not rule out existence of HDM. In fact, it is assumed that the dominant fraction of the dark matter is CDM type. But observations only tell us that $23\%$ of the total mass-energy of the observable Universe dark matter. But this information does not tell how much of it is HDM and how much is CDM? The belief is that the the dominant component of the dark matter is CDM but is there an estimate? Is there a physical way to understand/expect why the CDM should be the dominant contribution?
Please note There is a typo. I wanted to ask "Is the possibility of hot dark matter totally ruled out?"