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As other commenters have noted, the idea that dark matter could be composed of undiscovered black holes has been proposed before. The idea is that there might

As other commenters have noted, the idea that dark matter could be composed of undiscovered black holes has been proposed before. The idea is that there might be somewhat more generally "massive compact halo objects" (MACHOs), which would be at least as massive as planets, but would not emit light. A black hole would be one kind of MACHO, but neutron stars and free-floating planets would also be MACHOs as well. The issue is that these MACHOs would be detected through microlensing. If the conditions as right, as they pass in front of more distant stars, their gravity focuses the light from the background star, leading to an apparent spike in the star's brightness. By monitoring the sky for these microlensing events, astronomers have built up quite strong statistics about the presence of MACHOs in the Galaxy and have found that they are far too rare to comprise a significant component of dark matter.

T-A 10y ago
As pointed out in [1]: There is a window for PBHs to be DM if the BH mass is in the range 20M_sun<~ M <~ 100M_sun. Lower masses are excluded by microlensing surveys. Higher masses would disrupt wide binaries. [1] https://arxiv.org/abs/1603.00464 https://arxiv.org/abs/1603.00464
- ISL 10y ago
  The surprising thing to me, as a practicing professional physicist, with astrophysics connections, is that the MACHO hunts stopped without closing that window. It's such a clean explanation, and so many resources have been devoted to the hunt for particle dark matter while MACHOs still had a chance. In the late 90's/early 2000's, the paradigm was "WIMPS vs MACHOs", and then the MACHOs dropped off the radar following the lensing studies. When the paper you've cited appeared, my reply to the friend that pointed it out was, "Oh, there's all those lensing studies that convincingly sunk the MACHOs more than a decade ago, can't be true." When I went back, read the lensing papers, and realized that there was still a window, I felt really let down by the astrophysics community at large. What existing physics only gravitates and is intrinsically invisible? A black hole. They may not be the dark matter, but we owe it to ourselves to be real sure that they're not.
  - jessaustin 10y ago
    Is this a case of searching under the lamppost? As in, since lensing didn't demonstrate MACHOs, and there wasn't another obvious way to find them, maybe we should just look for something else? If so, that seems defensible on practical grounds?
    - ISL 10y ago
      As I understand it, the reason that the MACHO surveys weren't continued was because the event rate was low (you expect 1% the event rate with 100 M_sun MACHOs than 1 M_sun MACHOs). Looking longer would solve the problem. The thing I'm really bummed about is that MACHOs dropped out of the conversation ten years ago, so nobody new to the field would have known without digging that MACHOs still had a shot.

T-A 10y ago

As pointed out in [1]: There is a window for PBHs to be DM if the BH mass is in the range 20M_sun<~ M <~ 100M_sun. Lower masses are excluded by microlensing surveys. Higher masses would disrupt wide binaries. [1] https://arxiv.org/abs/1603.00464 https://arxiv.org/abs/1603.00464

ISL 10y ago
The surprising thing to me, as a practicing professional physicist, with astrophysics connections, is that the MACHO hunts stopped without closing that window. It's such a clean explanation, and so many resources have been devoted to the hunt for particle dark matter while MACHOs still had a chance. In the late 90's/early 2000's, the paradigm was "WIMPS vs MACHOs", and then the MACHOs dropped off the radar following the lensing studies. When the paper you've cited appeared, my reply to the friend that pointed it out was, "Oh, there's all those lensing studies that convincingly sunk the MACHOs more than a decade ago, can't be true." When I went back, read the lensing papers, and realized that there was still a window, I felt really let down by the astrophysics community at large. What existing physics only gravitates and is intrinsically invisible? A black hole. They may not be the dark matter, but we owe it to ourselves to be real sure that they're not.
- jessaustin 10y ago
  Is this a case of searching under the lamppost? As in, since lensing didn't demonstrate MACHOs, and there wasn't another obvious way to find them, maybe we should just look for something else? If so, that seems defensible on practical grounds?
  - ISL 10y ago
    As I understand it, the reason that the MACHO surveys weren't continued was because the event rate was low (you expect 1% the event rate with 100 M_sun MACHOs than 1 M_sun MACHOs). Looking longer would solve the problem. The thing I'm really bummed about is that MACHOs dropped out of the conversation ten years ago, so nobody new to the field would have known without digging that MACHOs still had a shot.