So if dark matter might be “Fermi balls,” why isn’t the idea being tested?
Quite surprisingly, such a theory is readily available for testing. Remember, dark matter avoids the center of galaxies, but neither does it condense into stellar-sized black holes (we looked). So if it is little balls created in the Big Bang, then it is indistinguishable from Primordial Black Holes that have been proposed for decades. The fate of a eensy-weensy black hole, according to current theory, is to evaporate by the emission of Hawking Radiation. For the moment, let us believe in the existence of Hawking Radiation despite the complete absence of data. Then PBH that are smaller, say, than a pea would have evaporated by now. They also would have been seen by now, because the smaller the PBH, the more of them it takes to be 5X more massive than the galaxies, and the Space Station flew big chunks of plexiglas looking for microscopic tracks left by micrometeorites.
But could these PBH be slightly larger and still exist? Then they wouldn’t have to be so numerous, wouldn’t emit Hawking Radiation, and we could have just not seen them yet?
The answer is yes. I just happened to write a proposal on it 8 years ago. If they are smaller than asteroids and bigger than peas, then they might have gone undetected. The larger and smaller PBH regions have been excluded by measurements. The technique is “nano-lensing” where gravity of a black hole spreads light like a diffraction grating. We would look at very very bright “Gammaray bursters” that are so far away (other galaxies) that the light is plane parallel. When a nano-lensing object lies between us and a Gammaray burster, the light appears as a rainbow and can be easily detected with the GRO gammaray observatory. So the 20 years dataset of Gammaray bursters would be analyzed for rainbows. We could exclude sizes from peas to houses pretty easily. With some work maybe up to asteroids.
Needless to say, the study remains unfunded.
You may also wish to read: Is dark matter the Fermi balls forged in the Big Bang? Physicist: Dark matter — the mysterious substance that exerts gravity but doesn’t interact with light — might be made of tiny black holes permeating the universe.