4.6.1. Normal Baryonic Dark Matter
An important feature concerning baryonic dark matter is that it wasn't necessarily always dark. A stellar mass black hole is the prime example as it once was a star radiating energy. Below we list some baryonic dark matter candidates. The local space density of these candidates will be calculated by assuming that the halo of our Galaxy is 1012 M and has a radius of 40 kpc. Since the Zaritsky et al. (1996) results suggest a halo size considerably larger than this, our estimates could easily be too high by factors of 10-100.
Bricks: A brick is an excellent candidate for dark matter as it has very high M / L. Assuming a mass of 1 kilogram per brick, the required space density is 1028 pc-3. The average spacing between bricks is 60,000 km and we would expect Earth to be impacted from time to time by a brick. The other problem with bricks is that they are made of heavy elements which are synthesized inside stars. Production of the dark matter brick population necessarily is preceded by a luminous phase of heavy element production in stars. The cosmic abundance of heavy elements provides a measure of all the light that has been produced in all the stars.
Small Balls of Hydrogen. A Jupiter mass ball of hydrogen would have high M / L in the optical bands. The mass of Jupiter is 10-3 M and the required space density is 10 pc-3. which only contributes 0.01 M pc-3 of mass density in the solar neighborhood. The nearest of these objects would be 0.4 pc away which would place it near the Oort cloud. Over the history of the Solar System, one might expect significant interactions between these interstellar Jupiters and the material in the Oort cloud. Moreover, although Jupiter will never be star is does radiate significant amount of energy in the 2-10 micron region of the spectrum. If this spectral behavior is similar for all Jupiter mass balls of hydrogen then for distances less than 1 pc, that flux would be easily detectable from earth based observations. Existing 2-micron surveys of the sky as well as the IRAS 12-micron survey have detected no candidate interstellar Jupiters. This is consistent with the microlensing surveys described below which have now placed stringent limits on the space density of these objects.
Stellar remnants: Since the Galactic disk is not yet old enough for white dwarfs to cool down to temperatures below a few hundred K or pulsars to slow down and cease pulsing, the only viable high M / L remnant that would escape local detection is stellar mass black holes. Assuming a mean black hole mass of 3 M requires a space density of 10-2 pc-3 which is pretty low. However, the seed population of these black holes are stars with masses 10 M. Those stars are the principal metal producing stars in the Galaxy. If they left behind 1012 M of black hole remnants then those remnants would have had to take their metals with them in order to avoid the interstellar medium being riddled with say, gold. In fact, the general problem with appealing to stellar remnants as dominating the mass of galaxies is that, in the past, the remnants were bright and galaxies should have much more substantial luminosity evolution than is actually observed (see Lilly et al. 1995). Moreover, in this context it is clear that the dark matter problem is linked directly to the chemical evolution of galaxies. This forms an expectation that metal rich galaxies have more stellar remnants/dark matter than metal poor galaxies.