What If Dark Matter Is Just Black Holes?
Why Hacker News recommends it
HN people link it in dark matter and black hole threads as a clear summary of why black holes alone are unlikely to explain dark matter.
What it is
PBS Space Time asks whether dark matter could be made of primordial black holes. It walks through the observational constraints that limit this idea.
Recommended by 0 HN commenters across 0 different months, 2022 to 2025.
What Hacker News said
Cites irjustin · 2022-01 · on “Astronomers discover a strange galaxy without dark matter”
When you dive into it, there are problems with this theory. PBS SpaceTime did pretty good coverage of it[0]. Each non-dark matter theory (i.e. MOND, primordial blackholes effort) usually has some pretty large glaring issue that's unsolvable. I'm all game to refute dark-matter but still waiting for the contender that fits the observations[1]. [0] [this video] [1] [LINK]
Cites ramraj07 · 2022-04 · on “I stopped working on black hole information loss”
Got it. The reference is great, the only final addendum I wanted to mention was whether a neutron stars densities would stop a black hole in its tracks since the paper only talks about the black hole already having been there. (At least to remove the amateur recollection of mine from the reference link, the mention of this possibility happens in the video at 6:18 [this video] ) and here’s them talking about a BH punching through earth which seems roughly in line with what you say as well! [LINK]
Cites irjustin · 2024-06 · on “Gravity Without Mass: UAH Study Proposes Alternative to Dark Matter”
This was covered by PBS spacetime: [this video] Still possible, but increasingly unlikely due to the narrowing size range in which these black holes are allowed to exist in.
Cites ck2 · 2024-09 · on “PBS SpaceTime: The New Physics of Black Hole Star Capture [video]”
(sorry only available on youtube, they no longer seem to mirror on pbs.org ) If you aren't familiar with Matt, yes he is at least 50% Neanderthal but also brilliant, lol btw I owe @api an apology, apparently black holes could indeed be dark matter, I missed this one from Matt a few years ago (or forgot the lesson) [this video] (actually by the end of that video Matt demonstrates point by point that there aren't enough black holes to be dark matter for several reasons/types, their mass doesn't add up enough, there would have to be more micro-sized black holes than all the stars in the universe and we'd be observing some really strange behaviors if that was the case, like disappearing neutron stars
Cites nabakin · 2025-04 · on “Astronomers confirm the existence of a lone black hole”
That was my first thought too. A cursory search indicates we would see a lot more gravitational lensing in our observations if that was the case. Found a couple of videos on it too [this video] [LINK]
Cites jraines · 2025-07 · on “Early universe's 'little red dots' may be black hole stars”
This PBS Spacetime video pours some cold water on the black holes as dark matter hypothesis: [this video] It’s 4 years old; I don’t know if this JWST finding changes anything. I do know we have finally found some (one?) intermediate-mass black holes in the interim, but I don’t know if that changes it either. The possibility it leaves open for “Planck relics” is interestingly exotic