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Wtf is dark matter. There's something out there that makes gravity not work the way we expect on a very large scale, and "dark matter" is a theoretical substance that makes the math work out properly. But the fact that such a huge portion of the galaxy's mass is this hypothetical, undetectable thing makes it seem very hand wavy. The last experiment to try to detect dark matter that I'm aware of concluded with "we successfully didn't detect anything" ๐ having to deal with dark matter feels like trying to study atoms before the discovery of the neutron. I hope we figure this out in my lifetime.
Dark matter might not even exist, all we know is that gravity-based predictions break down after a certain point. Dark matter is the just the most popular proposed solution where you essentially just add extra undetectable mass until it works. The distant second is Modified Newtonian Dynamics (MOND) or some variation of it, which is where you try to tweak the theories to fit observations instead. It has the same problem as dark matter where we keep coming up with better experiments which always fail to find anything.
There's a similar problem at the opposite end of the scale spectrum too; quantum mechanics doesn't play nice with our current understanding of gravity leading to the search for the "theory of everything". This is why I personally lean towards the idea that it's our theories that are wrong and not an undetectable mass, but this isn't my field so my opinion isn't worth much (especially since a majority actually working in the field lean towards dark matter as far as I can tell).
So in other words, the big equation of gravity gives us a formula on one side, and the solution + x on the other, and we have to solve for x (dark matter) but we don't know how to do it yet
I think I was listening to the unexplainable podcast and they were suggesting that gravity may work differently at smaller scale. Like the nooks and crannies may have different dimensional properties at the atomic or subatomic level. I dunno if I explained that right but it definitely got me curious. Like, we observe gravity as it effects large quantities of mass so, like temperature, it's really just an average of all the different factors at play.
And dark energy for that [dark] matter ;)
We know something is out there; galaxies are rotating far too quickly for our understanding of gravity to be correct. This is based on the observable matter.
For the galaxies to be rotating at the speeds we observe, we need approx 5 times the matter we see. So it is not like we have missed 10 - 20% of the matter that interacts with electromagnetic radiation, we would have had to have missed an extra 500%
As someone else pointed out, MOND is the next most promising candidate, but it has major issues even explaining what we see. Which is why it hasn't received widespread acceptance.
I don't have an answer; I have a few ideas. It maybe that something MOND adjacent is the answer; i.e. on the largest scales spacetime "relaxes" more when there is nothing pulling on it. So near galaxies and clusters spacetime is under more stress, this stress could equate to spacetime curving more on galaxy sized scales. But on the small scales we work on the extra stress will be almost invisible.
But as for us figuring out what "dark" matter is in your lifetime, unless you are already in your 80's; I think there is a very good chance. The only thing we know for sure about dark matter, is that it interacts with gravity (spacetime). We are building some pretty epic gravitational wave detectors, bringing the detection threshold lower.