[drewg123]

We know how stars work, and we know how gravity works...These two numbers don’t match, and they don’t match spectacularly. There had to be something more than just stars responsible for the vast majority of mass in the Universe.

I think the problem here is that maybe there is something else we don't understand. We also did not understand the propagation of light in a vacuum in the 19th century, and invented a hidden medium called the "aether" to explain it. I think there is some fundamental thing we are missing, and we're inventing dark matter to explain it.

[gbrown]

Perhaps we could call that thing: "dark matter" for now.

In seriousness, we can observe the effect, and even map the distribution of the stuff.

[sonnyblarney]

"In seriousness, we can observe the effect, and even map the distribution of the stuff."

No, we can measure the dissonance between our best theories and reality. And make a conjecture about 'stuff'.

[gbrown]

I'm not a physicist, but my basic understanding of the state of the science tells me you're simply incorrect. We can observe the effect of dark matter via gravitational lensing, and folks much smarter than me have devised studies to exclude differential behavior of gravity at classical and galactic scales.

Give me a recent and well supported study indicating that dark matter may be something other than matter, and I'll happily read it.

My understanding is that the existence of dark matter is uncontroversial among physicists, and that the best bet is some sort of WIMP.

[reitanqild]

As a non-physicist I assume the physicists have thought really hard about this.

But it feels very strange to accept something that:

- is a particle

- doesn't interact with other normal particles (except through gravitation)

- doesn't interact with particles of its own kind

[roywiggins]

Neutrinos are almost as strange. They interact a tiny bit via the weak force, and they interact via gravity like everything else, but that's it. We know they exist because of the weak interaction.

Maybe dark matter interacts with things via the weak force, but even less than neutrinos- not quite zero, but just small enough that we can't measure it, or can't measure it yet. Hopefully, they do interact a little bit by some other mechanism otherwise they'll be very hard to detect- I believe this is what current attempts to find dark matter are relying on.

[tylerjwilk00]

This assumes that we are 100% certain that our understanding of gravitational lensing is accurate _AND_ that only mass affects gravitational lensing.

[gbrown]

Nothing in science is assumed with 100% certainty. I would argue that working from our best available theories (while always looking for ways to falsify them) is perfectly reasonable.

[drewg123]

That assumes that it is "matter". Maybe there is something we just fundamentally don't understand about gravity.

[dTal]

They've thought of that: https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics

It doesn't really work. Overwhelmingly the evidence points to mass, which is gravitationally affected by other mass, and nothing else.

I know the mind rebels against the notion of invisible matter, but is it really so much more implausible than the invisible spookiness of gravity to begin with? If the scientific evidence points that way, your monkey-brain intuitions do not provide a reliable veto.

[bmiranda]

MOND does work a lot better than Lambda Cold Dark Matter (L-CDM)... but only for explaining the rotation of galaxies. It can explain this rotation almost perfectly given only the mass distribution: no tuning is required. Dark matter, on the other hand, has issues with dwarf galaxies and has no predictive power: you just fit the dark matter to the results you observe (Which is why you end up with some dwarf galaxies that are almost entirely composed of dark matter, and ones that almost have none. With MOND it just works).

Of course MOND can't really explain the third peak of the cosmic microwave background radiation, so it isn't perfect either. It is also phenomenological, with no underlying physical theory at the moment. Still, it's surprising that it does work at all.

I should also mention that the bullet cluster, which is touted as proving dark matter, causes issues for dark matter as well as MOND. The velocities involved in the collision are higher than can be explained by the current dark matter models. MOND kind of sucks at dealing with clusters, as well.

TL;DR MOND is a lot better than LCDM at explaining galaxy dynamics, LCDM is a lot better than MOND at explaining the cosmic background radiation. Both aren't that great at dealing with clusters (but you can also make dark matter work with enough fiddling).

https://tritonstation.wordpress.com/2018/09/04/dwarf-satelli...

[auntienomen]

Of course, neutrinos are invisible, and no one seems to have much of a problem with them anymore.

[improbable22]

This is another possibility, and would be exciting.

But it's very very hard to come up with plausible models for such modified gravity. There are lots of people trying, and their ideas tend to break all sorts of other things we know. For example (IIRC) lots of candidates gave a slightly different speed for gravitational waves, and were ruled out by recent detections where we also saw X-rays from the same event.

[_ph_]

Sure, but all data we have rather points out to some undetected matter than problems with the theory of gravitation. The article shows several measurements which would be well explained with particles we did not detect yet.

[gbrown]

That's not impossible, but I tend to believe the experts when they say it's not likely (and have explicitly ruled out several such explanations).

[ozmbie]

Who knows what future physics holds (I’d love to know!) but our best theory of gravity (Einstein’s general relativity) is shown to be amazingly accurate in every experiment ever conducted for it.

Some form of only-detectable-via-gravity matter fits the math. Coming up with provable alternative math would be the greatest scientific breakthrough of the past century...or possibly of all time.

I’m sure people are working on it (along with quantum gravity). So either it’s wrong or it’s dark matter.

[rleigh]

Maybe our assumptions about how momentum, gravity and inertia work are incomplete. General and special relativity describe it as a warping of spacetime, but while I've read all sorts about the equations, I have not yet read any description of how that phenomenon physically manifests (I'm not a physicist). Maybe relativity works well enough, but doesn't describe things completely, just like Newton's laws weren't complete and were superseded by relativity.