From the June 2014 issue

Could the mass of all the black holes and planetary bodies be a significant portion of the missing dark matter in the universe?

Colt de Wolf, Seattle
By | Published: June 23, 2014 | Last updated on May 18, 2023
dark matter mass
Primordial black holes, those that formed when too much energy gathered within a certain small volume just after the Big Bang, are created from radiation and not normal matter. That means that if these objects exist, they could make up some of the missing dark matter.
Marcelo Alvarez, John H. Wise, and Tom Abel
We know the total amount of material made of atoms is around one-fifth of the total amount of dark matter, the invisible mass in the universe. So nothing that is made of atoms, or that ever was made of atoms, can be a significant portion of dark matter. This rules out planets, dim and dead stars, etc., which are made of atoms. This result comes from many astronomical measurements using different techniques: the remnant microwave glow that fills the cosmos, deuterium (aka heavy hydrogen) abundances in distant galaxies, and direct searches for these objects.

All known black holes formed from stars at the end of their lives, so the mass in these black holes came originally from atoms. Thus, the amount of mass in ordinary black holes is tiny, and these objects cannot be a significant part of dark matter.

In the early 1970s, however, Stephen Hawking proposed that black holes could form from radiation in the extremely early universe, just after the Big Bang, when too much energy gets within a certain small volume that has a radius called the “Schwarzschild radius.” Because these primordial black holes (PBHs) formed from radiation, they are not made of atoms. Thus, PBHs could make up all, or a portion of, dark matter. There is no strong reason, however, to believe that these PBHs should have formed, so they may not exist — but it is a possibility.

Many experiments have looked for these PBHs, but they haven’t found any, and many of the possible mass ranges have been ruled out. However, if all PBHs have masses less than a tenth of the Moon’s mass, then these objects are still a possible dark-matter candidate.

Kim Griest
University of California, San Diego