>>13705585>Suppose these old white dwarfs are the dark matter: >>13705537 they became darker as the universe aged effectively making it as if early galaxies had no dark matter. White dwarfs have been ruled out as a significant contrition to dark matter. Compact objects cool were the first explanation for dark matter, coming from Zwicky who coined the term in the the 30s. In the late 80's a way to search for dark compact objects was devised using microlensing. Foreground dark bodies would occasionally lense background stars, you just had to watch a lot of stars. Huge surveys were conducted, and the result was only a tiny number of candiate events. The vast majority of dark matter cannot be made up by objects like white dwarfs and stellar black holes.
Also white dwarfs are composed of normal matter, and cosmology indicates that there is nowhere near enough baryons to explain all of dark matter. Two probes which constrain the density of normal matter are the fusion in the early universe and the fluctuations in the cosmic microwave background. Both tests are probing times before stars, before galaxies, no white dwarfs. The CMB shows that even at 300,000 years dark matter was in place, not behaving like the normal matter.
Maybe one could imagine rewriting cosmology to get around the later constraints, but there's no guarantee it is possible. The microlensing constrain is just simple physics, it's not going away.
