Observations and models suggest that most of the matter of the universe is invisible. It doesn’t interact with light, and only makes its presence known with gravity. For this reason, it is called dark matter. There is another celestial feature that also doesn’t emit light: black holes. Researchers have long suggested that they are one and the same.
Now, a team of physicists from the University of California Berkeley has ruled this out, claiming that black holes cannot be the main component of dark matter. In a paper published in Physical Review Letters, they state that black holes that formed during the Big Bang can at most account for 40 percent of dark matter. An expansion on their analysis, which is yet to be published, puts the number as low as 23 percent.
The team analyzed 740 of the brightest distant supernovae to make this estimate (in the study currently in the works, the number is 1,048). The theory is if black holes – which are usually found at the center of galaxies – were the cause of dark matter, they would be scattered around galaxies and not just found in the center. Being such dense objects, they bend space-time and they can act as a gravitational lens, magnifying the light of background objects.
For dark matter to be black holes you would need a lot of them, and so the light of at least some of these supernovae should be affected. However, no changes in the brightness of their light were observed, indicating that black holes cannot be the main component of dark matter.
However, this doesn't necessarily clear things up, it may have actually made things worse. The strongest hypotheses had dark matter made of a single component, whether black holes or microscopic particles. Having dark matter made of several different things might make it even more complicated to explain how all the different components form.
“I can imagine it being two types of black holes, very heavy and very light ones, or black holes and new particles," lead author Miguel Zumalacárregui, a Marie Curie Global Fellow at the Berkeley Center for Cosmological Physics, theorized in a statement.
"But in that case one of the components is orders of magnitude heavier than the other, and they need to be produced in comparable abundance. We would be going from something astrophysical to something that is truly microscopic, perhaps even the lightest thing in the universe, and that would be very difficult to explain."
The mystery of what dark matter is still remains, but at least we know that we can probably rule out black holes as the main cause behind it.