Likelihood of MACHOs being the best candidate for dark matter

Question

Massive compact halo objects ("MACHOs") include a wide variety of hardly detectable bodies such as brown / white / black dwarfs and black holes, to name a few. If we take into account the inevitable end of all stars into either a white dwarf, a neutron star or a black hole and we compare the average lifespan of a star with the time star formation has been active since the Universe started, then we can tell galaxies already contain many dead stars which still have mass and can therefore interact gravitationally with the rest of the bodies in said galaxy.

Furthermore, stars tend to accumulate in the galaxy's bulk (primarily) and in the galactic disc as well, so if dark matter happened to be MACHOs, which are distributed in said proportions, then this would account for the fact that models such as the dark matter halo need to have a density such that it decreases the further away we get from the centre of the galaxy.

Finally, MACHOs are bodies we know to exist for a fact, whilst particle dark matter (e.g. WIMPs) has not been found. Is there a reason for which MACHOs are not the most likely candidate for dark matter?

Answer 1

Due to the statistics of the temperature fluctuations in the cosmic microwave background and the abundance of light elements/isotopes that emerged from primordial nucleosynthesis, we know that the dark matter is nonbaryonic. Specifically, the cosmic microwave background tells us both the total abundance of matter (e.g. both dark matter and baryons) and the abundance of matter that was strongly coupled to the primordial plasma (e.g. baryons), while nucleosynthesis tells us the abundance of baryons. Both are consistent with baryons comprising about 5% of the energy density within the present-day Universe and much more weakly coupled nonbaryonic matter comprising about 25%.

This means that the dark matter cannot be brown dwarfs, planets, and such, since they are baryonic. The dark matter also cannot be stellar remnant black holes, since the mass that formed them was just baryons in the primordial universe.

However, the dark matter could be nonbaryonic MACHOs. For example, black holes that formed in the primordial universe are a viable dark matter candidate. There are observational constraints on their abundance in a variety of mass ranges, but not all. Some people are rather optimistic about primordial black holes.

Answer 2

I'll add a couple of points to Sten's answer.

1. MACHOS were taken seriously as dark matter candidates for a while. Gravitational microlensing surveys took place from the 1990s onwards to place constraints on the numbers of MACHOS (in the halo). Conclusion: MACHOS of a planetary to stellar mass ($10^{-7}< M < 30$ solar masses) make up less than 40% of the Galactic dark matter (and less than 25% in the planetary regime, Alcock et al. 1998; Alcock et al. 2001). Or, more recently, that $<15$% of the dark matter halo is in the form of compact objects with masses between $10^{-6}$ and 100 solar masses (Blaineau et al. 2022).

2. The Galactic dark matter needs to be distributed as a halo of decreasing density around the Galaxy, but much more extended than the distribution of visible stars. If the dark matter were compact stellar remnants it seems hard to understand why it should be distributed differently to the old halo stars that were formed at the same time as the Galaxy.