Comment by GuB-42
10 hours ago
I think there is a shortcut being taken here.
We are surrounded by dark objects, a rock is a dark object, exoplanets are dark objects, and so are black holes. Pretty much everything but stars are dark objects. They are all dark because they don't emit light.
Here, I think they mean stuff (whatever it is) that can only be detected by gravitational lensing, and it makes sense that it has to be extremely heavy, because gravity is so weak.
I'm not a physicist but every definition of dark matter that I read says it does not interact with electromagnetic radiation hence it is invisible, and rocks are not that dark matter (wiki. NASA, etc)
So how do we know that these "dark matter objects" aren't actually just massive collections of normal matter that is dim enough and at such a far distance that it would appear (angular resolution-wise) to be invisible, but we can still detect the lensing?
> just massive collections of normal matter
Normal matter in the universe is mostly hydrogen, which should coalesce to form stars, which give off light. The lack of light compared to the estimated mass is precisely the paradox.
There are a few reasons. It would be visible when backlit. Gravitational lensing detection limits the size so it can't planets (MACHOs). The CMB shows that only sixth of matter interacts with other matter, the rest is only interacts gravitationally.
> ... every definition of dark matter that I read says it does not interact with electromagnetic radiation ...
Actually, dark matter does interact with electromagnetic radiation -- it can deflect it, as in the case of gravitational lensing. But dark matter doesn't either emit nor absorb electromagnetic radiation directly.
We only know about dark matter because of its gravitational effects.
How about stellar mass black holes?
They are much lighter than 1 million solar masses and we know a few of them, with a variety of ways to detect them, including companion stars orbiting around them and gravitational waves during mergers.
Black holes fit the definition of dark matter, as they neither emit nor absorb electromagnetic radiation, not in a way that could be detected anyways. This is the "MACHO" theory of dark matter, which is not the favorite, but it is still taken seriously. Stellar mass black holes have been ruled out, I think, but it doesn't mean dark matter can't be made of black holes. In fact, primordial black holes are a rather hot theory.
yeah all those other things absorb light so they can be detected by the light they block and the infrared light the re-emit.
Dark matter seems more ghostly , like gravitational shadow of matter