The Scientific Method indicates that when observational evidence contradicts the theory, the theory is considered disproven and chucked. That’s not the case with dark matter: galaxies aren’t behaving as classic theories predict, so dark matter is postulated. And it’s not falsifiable. IOW it’s exactly like a kludge, a fudge factor.
So why are scientists so sure dark matter exists?
It’s the simplest assumption that fits the observed data. And it’s more than just the way galaxies act; for example, they can see gravitational lensing around something they can’t see. Dark matter is a more conservative assumption than assuming that spacetime is warped in just such a way as to look exactly like dark matter, without anything actually being there.
Clearly SOMETHING is out there that we can’t see. The observable universe doesn’t show the mass that is expected…in fact, IIRC, only a very small percentage of the estimated mass of the universe is actually visible. Since that seems to be the case, they merely call the mysterious mass we can’t see ‘dark matter’.
I think you are wrong about the falsifiability, however. You COULD falsify the existence of dark matter (or something that makes up the difference in the projected mass verse the amount we can see) by merely positing a theory as to why there really shouldn’t be as much mass as predicted, and that the prediction itself is wrong…and then having some way to demonstrate that with data.
Clearly? Why do we think that current theories are correct?
That’s a circular argument, though, isn’t it?
That doesn’t make it right.
I would say that scientists are not at all sure that dark matter exists.
They are sure that effects of something like dark matter are observed, and that the existence of dark matter is probably the simplest explanation for those observations.
Essentially “dark matter” is a placeholder term to explain the observations.
Yes, clearly. We can see its effects. Either there’s matter we can’t see out there, or some effect that duplicates the exact effects of matter that we can’t see.
No, but it makes it the most plausible theory as of this time. Science doesn’t do “absolute certainty” if that’s what you are looking for.
[QUOTE=Quartz]
Clearly? Why do we think that current theories are correct?
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Sadly I’m not a physicist or cosmologist, so you are going to get the Science Channel version as filtered through my own understanding so…GRAIN OF SALT ALERT. Basically, the mass observed in the universe, or in individual galaxies is less (like 80-90% less IIRC) than the gravitational effects. That’s not a rounding error, but a huge discrepancy. SOMETHING has to be causing the gravitational effects we are able to observe, but the observable matter (stars, dust, gas, etc) doesn’t add up. Thus, ‘dark matter’.
Not as far as I can tell. Explain why the gravitational effects observed match, within a reasonable amount the observed matter and you’d disprove that we need ‘dark matter’ to make up the huge difference. Or explain and demonstrate something else that’s accounting for the gravitational effects observed.
Afaik, a Nobel prize is in store for anyone who does.
Basic question here: Does “dark” mean “imperceptible”? I.e., is it possible that I am sitting next to – or even, inside – a huge mound of dark matter, but I can’t see it or feel it because dark matter does not interact with ordinary matter, except gravitationally? Or, are we talking about something that is ordinarily perceptible, but we don’t perceive because it’s all in tiny clumps or atoms scattered around interstellar space? Or what?
And absolutely all of the evidence points to the fact that dark matter and dark energy make up the vast majority of Universe. I’m not sure how you could possibly argue otherwise if you’ve been reading the literature.
That’s absolutely incorrect. We can measure the mass that causes gravitational lensing and we know for a fact that the observable matter does not and can not account for it. Just as one example.
Bull.
You simply need to perform experiments that show that the observable mass-energy of Universe accounts for what we see happening in Universe.
Good luck.
Dark Matter is falsifiable. There was significant debate between whether what we saw was the result of fundamentally misunderstanding F=MA at large scales and that gravity wasn’t constant at all scales. But since then we’ve made observations that suggest dark matter is a discrete substance of some sort, rather than a misunderstanding of the laws of the universe. It clumps. These clumps can cause light to lens around them due to their gravitational effect. It also can be split up unevenly (not in line with the visible matter) when some sort of cosmic structure is split up or collides.
Science is very much studying if dark matter is a “fudge factor” to cover our fundamental misunderstanding of the laws of the universe, but more evidence points to it actually being a discrete, independent… something out there.
Dark matter could be ordinary matter that’s just plain dark, like a bunch of iceballs, rockballs, or gasballs wandering interstellar space. Or it could be things like black holes, or other forms of exotic matter but that we’ve at least thought of. Or it could be something like neutrinos, but with a tiny mass instead of zero mass. Or it could be something we haven’t even thought of. Or it could be that it doesn’t exist, and out theories of gravity are wildly wrong.
We’re not sure. WIMPS are one of the leading hypothesis - essentially some sort of weird particles that don’t react electromagnetically but still have gravity. So yeah, there could be WIMPS and dark matter all around us, but not in a way that we can observe or that affects us outside of gravitation.
OK.
[performs experiment]
Hmm . . . No matter how or in what units I measure the results, the data always come out reading “42.” 
Some of it could, but as I understand it there’s too much of it for it all to be normal matter. There’s apparently a theoretical upper limit on the abundance of normal matter, because a greater abundance would have affected the creation of elements in the early universe in ways we’d be able to see.
So whatever is out there, most of it is to believed to be made of some other kind of matter than what we are.
The reason we believe that there is dark matter is because everything that we observe behaves as if there were dark matter. That’s the same reason I believe there’s a wall opposite me: if there were in fact a wall, I would expect myself to make certain observations; I do make those observations, and thus, I accept the wall hypothesis.
With respect to dark matter, it was introduced in order to explain certain discrepancies with the observed rotation curves of galaxies. One possible explanation for this is the introduction of unobserved additional matter. If there were such dark matter, we would observe, for instance, certain gravitational lensing effects—which we do, in fact, observe.
Now, there’s other possible explanations that are also being considered, such as Milgrom’s modified Newtonian dynamics or MOND, which can explain the rotation curves as well; however, these hypotheses are less well in accordance with other observations, such as the behavior of galaxy clusters, etc.
So basically, nobody forces you to accept the dark matter hypothesis: you’re free to come up with an alternative. However, it has to fit observational evidence at least as well as dark matter does—and there’s just nothing on the table at the moment.
Well, nothing we can see directly, anyway.
I understand dark matter is the simplest theory that we have bolted on top of our accepted theory about everything else.
But if you want to take the Ocham’s Razor approach then I’d argue that bolting lots of simple ideas on top of each other no longer delivers the simplest solution. Instead we should start from scratch. More simple solutions for some of these observations are ideas such as a diminishing speed of light.*
No, I think positing ‘dark matter’ is actually the simplest explanation, not the most complex. That doesn’t necessarily mean it’s right, but it passes the Ocham test anyway.
[QUOTE=BrainGlutton]
Basic question here: Does “dark” mean “imperceptible”? I.e., is it possible that I am sitting next to – or even, inside – a huge mound of dark matter, but I can’t see it or feel it because dark matter does not interact with ordinary matter, except gravitationally? Or, are we talking about something that is ordinarily perceptible, but we don’t perceive because it’s all in tiny clumps or atoms scattered around interstellar space? Or what?
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No one knows for sure, since they know next to nothing about ‘dark matter’ (or ‘dark energy’ which is something entirely different, though they both have ‘dark’ in their description :p). All that’s known is the effects they have. ‘Dark matter’ is the name they call whatever is making up a large percentage of the mass of the universe that we can’t see. ‘Dark energy’ is the force(?) that causes galaxies to move apart faster than predicted and the universe to expand more rapidly than predicted. That’s my Science Chanel take on it anyway, and afaik that’s about all that’s known. I saw a show on Science once that someone (NASA?) had discovered what they thought was some evidence about ‘dark matter’, but I’ve found that you have to take those shows with a grain of salt (plus my memory is often flawed), so no idea if it ever panned out.