Dark Matter Question

[Turtle Shed]

I enjoy the Sea channel on Youtube: https://www.youtube.com/@sea_space

In his video about Dark Matter he tells how it is (so far) undetectable because it doesn't interract with anything. Scientist are very confident that it exists though, or at least that *something* exists to account for the configuration of the universe. 5/6ths of all matter speculated to be Dark Matter (or "missing mass" as he describes it).

My question then is how is this stuff so undetectable if it interracts with entire galaxies to shape the universe?

[Eric Mc]

Dark matter is deduced through its gravitational effects. So it does interact with other matter in the universe. in fact, it's the interation that suggests it exists in the first place.

[Turtle Shed]

Eric Mc said:

Dark matter is deduced through its gravitational effects. So it does interact with other matter in the universe. in fact, it's the interation that suggests it exists in the first place.

Yes, that's what I can't get my head around. Gravitational effects on entire universes but doesn't intereact with anything that we can throw at it.

[Mr Pointy]

It's a fudge. The calculations don't add up so they have had to invent something that makes them work.

[deckster]

Mr Pointy said:

It's a fudge. The calculations don't add up so they have had to invent something that makes them work.

To an extent you're right, but it would be more correct to say that our observations of the universe don't match up with what we think we understand about the matter that we can see; the way that galaxies form and interact suggest that they are far more massive than they appear. The existence of dark matter is largely confirmed by the gravitational lensing effect that we from far-distant galaxies when there, apparently, isn't any intervening mass for the light to be lensed by.

It's is also incorrect to say that the postulated dark matter doesn't interact with anything. It doesn't emit light or any other kind of radiation, that we can detect, but it definitely interacts on the gravitational level.

The fact that we can directly observe its effects means that we know it exists, and it's not a fudge. We just don't know exactly what it is.

I would have more respect for the initial statement if we were talking about dark energy, which is a completely different beast. This I would concede could reasonably be classified as a fudge, in that we can't observe it, can't see any effect that it has, but need "something" to explain why the expansion of the universe appears to be speeding up.

As I say, dark energy and dark matter are completely different things and it is a shame really that they were given such similar names. But for sure, dark matter can be observed if only indirectly and definitely exists.

[Toltec]

I see it as a place holder until we can explain what is causing space to appear as if there is far more mass in it than we can see.

[Panamax]

deckster said:

dark energy and dark matter are completely different things and it is a shame really that they were given such similar names. But for sure, dark matter can be observed if only indirectly and definitely exists.

Yes indeed. It would be better if "dark matter" was simply called "invisible matter", because there's nothing dark about it.

Similarly "dark energy" might more usefully be called "unknown energy".

Dark/invisible matter is known because of it's measurable gravitational effects. On the other hand dark/unknown energy is simply a term to describe an apparent effect that has no explanation whatsoever.

Interesting article here about black holes' possible interaction with these concepts,
https://physicsworld.com/a/new-theory-links-superm...

I console myself that if weird things like gravity, magnetism and electricity can exist then it's not particularly surprising if there's other invisible stuff out there.

[Turtle Shed]

All very interesting, and maybe I'm getting his videos on dark matter and dark energy mixed up.

It just feels to me (and I'm no scientist) that there's a hint of Victorian "Ether" or Einstein's "Cosmological Constant" at play here. Based on the comments above, it seems to be dark energy where that comparison is more appropriate.

This actually raises something I have often wonder about. I remember that the Victorians thought they knew almost all of how science worked, and that everything there was to know would soon be known. In other words, they didn't know what they didn't know. I wonder therefore if here in the 21st century, our scientists are more aware than the Victorians were at the limited extent of their knowledge.

[thegreenhell]

It sounds like a fudge to cover the fact that we don't understand gravity. We don't fundamentally know what causes gravity, and the equations we use to predict it don't work at the very small scale or the very big scale.

[Skeptisk]

deckster said:

Mr Pointy said:

It's a fudge. The calculations don't add up so they have had to invent something that makes them work.

To an extent you're right, but it would be more correct to say that our observations of the universe don't match up with what we think we understand about the matter that we can see; the way that galaxies form and interact suggest that they are far more massive than they appear. The existence of dark matter is largely confirmed by the gravitational lensing effect that we from far-distant galaxies when there, apparently, isn't any intervening mass for the light to be lensed by.

It's is also incorrect to say that the postulated dark matter doesn't interact with anything. It doesn't emit light or any other kind of radiation, that we can detect, but it definitely interacts on the gravitational level.

The fact that we can directly observe its effects means that we know it exists, and it's not a fudge. We just don't know exactly what it is.

I would have more respect for the initial statement if we were talking about dark energy, which is a completely different beast. This I would concede could reasonably be classified as a fudge, in that we can't observe it, can't see any effect that it has, but need "something" to explain why the expansion of the universe appears to be speeding up.

As I say, dark energy and dark matter are completely different things and it is a shame really that they were given such similar names. But for sure, dark matter can be observed if only indirectly and definitely exists.

I think it would be fairer to say that the majority of cosmologists think dark matter exists but there are others who are argue that general relativity is wrong and propose other theories of gravity eg MOND. I think the definite confirmation of the existence of dark matter will only come if someone can show another form of interaction with normal matter. There are a number of experiments that have so far failed to do that. However it took us about a century between Einstein’s prediction of gravity waves and an experiment that proved their existence so it might be some time yet.

[Panamax]

Turtle Shed said:

I wonder therefore if here in the 21st century, our scientists are more aware than the Victorians were at the limited extent of their knowledge.

That's a good point. They'd got the "physical world on earth" pretty well understood and had an understanding of plants/moons, but that was about it. It later transpired most of their rules only worked at normal temperatures and pressures on earth.

Then came a happy time when "the atom" was "the smallest thing". But then people started taking atoms apart and the discovery remains ongoing.

At the same time the religious beliefs about how things work were being pushed aside by scientific discovery.

As you say, once people accept "there's some stuff we don't know" it's not particularly surprising to find more and more stuff we don't know. The key mindshift is to accept it doesn't matter there's stuff we don't know - rather than revert to a fictional religious explanation.

[budgie smuggler]

thegreenhell said:

It sounds like a fudge to cover the fact that we don't understand gravity. We don't fundamentally know what causes gravity, and the equations we use to predict it don't work at the very small scale or the very big scale.

One proposal to solve this issue is Modified Newtonian Dymanics (MOND). As per the name, this is a tweak to Netwons law of gravity that makes calculations about galaxies fit observations. There are some issues with it however.

Personally I think WIMPs are the most likely candidate, something similar to neutrinos but more massive.

[Steve Campbell]

One thing to realise is that scientific theory is never fact.

The underlying principles of science suggest that a theory explains why or how something works, it's postulated and experiments derived to try and show that the theory is correct by producing facts or evidence that support the theory. Facts & evidence can be directly measured or inferred based on reasoning from what we see.

Even if that becomes so, it does not change the theory into fact, merely a well supported and evidentially shown to be true theory......until it is disproved by a later theory or the scientific evidence stacks up later to show what we thought wasn't quite right.

I find the whole "dark" discussion incredibly difficult to follow, but it helps me to understand it conceptually as above ie "we've spotted something we don't quite understand based on current theory, so we made up a new theory called "dark" that explains it...but we haven't yet conclusively proven that theory just yet"

[deckster]

Steve Campbell said:

One thing to realise is that scientific theory is never fact.

The underlying principles of science suggest that a theory explains why or how something works, it's postulated and experiments derived to try and show that the theory is correct by producing facts or evidence that support the theory. Facts & evidence can be directly measured or inferred based on reasoning from what we see.

Even if that becomes so, it does not change the theory into fact, merely a well supported and evidentially shown to be true theory......until it is disproved by a later theory or the scientific evidence stacks up later to show what we thought wasn't quite right.

I find the whole "dark" discussion incredibly difficult to follow, but it helps me to understand it conceptually as above ie "we've spotted something we don't quite understand based on current theory, so we made up a new theory called "dark" that explains it...but we haven't yet conclusively proven that theory just yet"

If you want certainty, go read the bible.

If you prefer an open exchange of ideas that propose models to fit our best current observations, that can be challenged, experimentally validated, and modified as and when new data becomes available then welcome, you're a scientist.

[Wills2]

Turtle Shed said:

All very interesting, and maybe I'm getting his videos on dark matter and dark energy mixed up.

It just feels to me (and I'm no scientist) that there's a hint of Victorian "Ether" or Einstein's "Cosmological Constant" at play here. Based on the comments above, it seems to be dark energy where that comparison is more appropriate.

This actually raises something I have often wonder about. I remember that the Victorians thought they knew almost all of how science worked, and that everything there was to know would soon be known. In other words, they didn't know what they didn't know. I wonder therefore if here in the 21st century, our scientists are more aware than the Victorians were at the limited extent of their knowledge.

Isn't that why explanations are called a theory, as the name suggests it's a theory based upon what we think we know and can observe, so it's open to discussion, challenge and research as we know more the theory can be developed.

[RizzoTheRat]

We have theories that explain most of what's going on, but there are often some anomalies. Usually several different alternative theories are proposed that could explain the anomalies, and then scientists look for ways to test those theories to see if one of them is correct. Dark matter and dark energy are proposed theories that would explain the observations.

This approach isn't new. In the late 1700's astronomers discovered Uranus. However it's orbit wasn't what they expected it to be. One explanation for this was the presence of another planet, and by the mid 1800's they managed to calculate where that planet should be...and discovered Neptune.

[Steve_H80]

RizzoTheRat said:

In the late 1700's astronomers discovered Uranus. However it's orbit wasn't what they expected it to be. One explanation for this was the presence of another planet, and by the mid 1800's they managed to calculate where that planet should be...and discovered Neptune.

One of those great "told you so" moments

[deckster]

RizzoTheRat said:

We have theories that explain most of what's going on, but there are often some anomalies. Usually several different alternative theories are proposed that could explain the anomalies, and then scientists look for ways to test those theories to see if one of them is correct. Dark matter and dark energy are proposed theories that would explain the observations.

Indeed, although I would make a difference between "theory" which is what scientists use when we're pretty sure we're broadly speaking right even if we can't explain all the details (evolution, quantum, gravity), and hypothesis which is something that we're just suggesting. Dark matter & especially dark energy are definitely not theories yet, and dark energy barely scrapes into the hypothesis category right now.

[RizzoTheRat]

Fair point in theory vs hypothesis, but the guy who came up with Dark Energy got a Nobel prize for it so I thought it was fairly well accepted?
He was on Infinite Monkey Cage some time back and reckoned he'd got the prize for reducing the total amount of human knowledge by finding such a massive unknown

[deckster]

RizzoTheRat said:

Fair point in theory vs hypothesis, but the guy who came up with Dark Energy got a Nobel prize for it so I thought it was fairly well accepted?
He was on Infinite Monkey Cage some time back and reckoned he'd got the prize for reducing the total amount of human knowledge by finding such a massive unknown

To be more accurate, the three who were jointly awarded the Nobel got it for the discovery that the universe was expanding faster than can be explained by the amount of matter (== energy) that we see in the universe. Dark energy is the name we give to the hypothesis that there is some invisible, unknown energy that is driving this expansion. There are other candidate hypotheses to explain it though, mainly involving weird things happening to gravity.

But either way this is a great example: our observation is that the universe is expanding faster than our current theories allow. Therefore something is wrong. We hypothesise a number of solutions that would explain what we observe. We test to see if any of the hypotheses are correct. This is literally how science works