It’s just shorthand for stuff that we cannot detect but ordinary (electromagnetic) means. In theory everything ‘feels’ gravity and anything with mass contributes to it. The left-handed neutrino is unusual in that it experiences only gravity and the extremely weak (though far stronger than gravity) “weak nuclear force” (and their right-handed brethren, if they even exist, might not exist en experience that).
The “dark sector” is that 95% or so of the entire universe we infer must exist for either gravitational reasons (stronger gravity leads us to infer unseen mass, which we term “dark matter” though “transparent and apparently frictionless” would be a better term, or to infer unseen energy (dark energy that seems to be driving the accelerating rate of the universe).
It's worth noting that there is an explanation for dark energy, namely quantum fluctuations of the vacuum. It's also worth noting that this explanation is off by over 100 orders of magnitude, which lead to it being called "the worst prediction in physics". Oddly enough we haven't come up with a better explanation as far as I know, the current assumption is just that the mass somehow almost cancels out.
> 95% or so of the entire universe we infer must exist
Isn’t the reason we infer dark matter exists, is our observations of the spin of galaxies? In other words there must be a lot more gravity produced by matter we aren’t observing (dark matter) to explain large scale observations?
Aren’t there competing theories to dark matter, which haven’t been ruled out, from Einstein’s theories of gravity being wrong; or in the same way Einstein’s theory of gravity breaks down/is incomplete on the quantum scale his theories may breakdown at the scale of galaxies; and even more simply our observations not be accurate?
Contrarian theories have outsized popularity here on hnews, and this tends to get fed by garbage articles from the usual sources (glances at quanta with mild disdain). Modified gravity theories are not some alternate path we just didn't explore. We explored them. They fail to explain the observational evidence. In particular the way stuff moves in galactic collisions makes very clear there's dark matter distinct from some modified version of gravity interacting with visible matter.
It ends up that the consensus view of thousands of Phds that study this stuff is actually not trivially mistaken.
I wasn’t implying mistaken by any means, I understand is the leading theory (not the only theory), I didn’t realize everything else had been ruled out or considered junk science (I just understood no one wants to be the person saying Einstein’s theories/equations for gravity need modification...even though that happens to be the consensus at the quantum level.
But the galaxy collision observations is something I want to read more about (I’ve obviously seen simulations), thanks!
To build on peer replies: the leading dark matter theories from a particle physics perspective imply "particles which do not feel electromagnetism." This ought not to be weird! After all, some particles do not feel the strong force (leptons) and others do not feel the weak force (right-handed); why should every particle feel EM?
It feels weird because almost everything we see is EM in nature: chemistry, electricity, light, life itself! But there is no a priori reason to demand that everything in the universe interacts through EM. There ought to be some stuff that does not. And if it were to exist, it would be like a ghost: particles which could not shed energy through emitting photons.
And the cosmological perspective is that "there's a lot of this stuff, huge amounts" and why not?
The “dark sector” is that 95% or so of the entire universe we infer must exist for either gravitational reasons (stronger gravity leads us to infer unseen mass, which we term “dark matter” though “transparent and apparently frictionless” would be a better term, or to infer unseen energy (dark energy that seems to be driving the accelerating rate of the universe).