Artistic representation of black holes in the halo of a spiral galaxy.
When gravitational waves were first detected at the LIGO interferometer, right after switching it on, the fact that they were produced in a collision of two black holes of about 30 solar masses each was surprising for scientists. It is uncommon for black holes formed by supernovae to be that big, because then the stars that exploded must have had large masses that only happen on very rare occasions and under special circumstances (the material that forms the star must have been almost only hydrogen and helium and barely anything else). However, the fact that they were detected so early suggested to some that these massive black holes might be more common than previously thought.
But there are some theories that predict that large numbers of black holes would have been created in regions of high density very soon after the big bang, and many would remain today. Some even proposed that these "primordial black holes" could be what dark matter is made of. And if there are so many as to be the main component of dark matter and they had this range of mass, they would explain the gravitational wave observation.
However, if millions of black holes are surrounding every galaxy, perhaps we could detect them with gravitational lensing, as their mass deflects light coming from a bright object further away. I recently participated in a study to do just that, and we concluded that these black holes cannot
form a significant fraction of the dark matter or there would be gravitational lensing effects that we don't see. The study was accepted by The Astrophysical Journal Letters, and it can be read here: arxiv.org/abs/1702.00947
Yesterday, a Spanish science popularizer specialized on Physics decided to write about it, and suggested that it would be good to have a good illustration to accompany the text. So I made this piece
Here is his post, in Spanish: francis.naukas.com/2017/02/10/…
The image was created using Cinema 4D, and a technique I developed about 9 years ago to make this video of a black hole in front of a planet: www.youtube.com/watch?v=grTczB…
The background galaxy is a picture of M81 by the Hubble Space Telescope, available here
under the Creative Commons Attribution 4.0 International license