After 90 years of searching for dark matter, astrophysicists have yet to find definitive evidence of its existence. A new hypothesis suggests that dark matter could be composed of miniature black holes.
These mini black holes, with a mass of about 22 micrograms, could have a quantum mechanical size defined by the Compton wavelength. This means that their behavior is governed equally by quantum mechanics and gravity, complicating our understanding of their fate.
Stephen Hawking proposed in 1974 that such mini black holes evaporate quickly due to thermal radiation. However, black holes with Planck mass might be stable, raising the question of whether they could constitute dark matter. This idea was first introduced by JH MacGibbon in 1987.
The interaction cross-section of Planck mass black holes is extremely small, making them viable candidates for dark matter. To account for dark matter, there would need to be one Planck mass black hole for every 10 27
For the dynamics of the Milky Way, dark matter must have a local mass density equivalent to 0.4 proton masses per cubic centimeter, suggesting that a Planck mass black hole passes through our bodies each year without posing a health risk due to their minuscule interaction cross-section.
These black holes could be remnants from early universe phase transitions or decays of heavy fields. While Hawking had suggested primordial black holes might be electrically charged, later insights indicated that such charge would lead to rapid neutralization.
Future studies may reveal more about these elusive entities. Recent findings show that gravitational wave observatories like LIGO-Virgo-KAGRA can detect signals from massive stealth spacecraft moving near light speed, and upcoming facilities like LISA will enhance this sensitivity.
However, if dark matter is indeed made up of very small black holes, direct detection may remain elusive.