Black holes probably not accounting for dark matter

One theory bouncing around for a long time has been that maybe primordial black holes make up a lot of the universe's dark matter.

However, a new attempt to find evidence for this has drawn a blank:

Based on a statistical analysis of 740 of the brightest supernovas discovered as of 2014, and the fact that none of them appear to be magnified or brightened by hidden black hole "gravitational lenses," the researchers concluded that primordial black holes can make up no more than about 40 percent of the dark matter in the universe. Primordial black holes could only have been created within the first milliseconds of the Big Bang as regions of the universe with a concentrated mass tens or hundreds of times that of the sun collapsed into objects a hundred kilometers across.

The results suggest that none of the universe's dark matter consists of heavy black holes, or any similar object, including massive compact halo objects, so-called MACHOs.

Dark matter is one of astronomy's most embarrassing conundrums: despite comprising 84.5 percent of the matter in the universe, no one can find it. Proposed dark matter candidates span nearly 90 orders of magnitude in mass, from ultralight particles like axions to MACHOs.

Several theorists have proposed scenarios in which there are multiple types of dark matter. But if dark matter consists of several unrelated components, each would require a different explanation for its origin, which makes the models very complex.

Personally, I'm still more inclined to suspect that it's gravity that needs modifying.