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Aseyhe t1_j46bqop wrote

Reply to comment by Interesting-Month-56 in How do we know that dark matter isn't just ordinary matter our instruments can't detect? by jmite

> These things tend to clump together and do things like fall into stars where we can detect them by their emission lines.

I think that requires some additional assumptions. Dark matter indeed couldn't be gas, because we can detect gas on its own, and also gas would lose energy and fall into things (whereas dark matter does not). However, high-density low-brightness objects made of ordinary matter (like free-floating asteroids or planets) would behave just like dark matter and would be incredibly difficult to detect. Granted, it would not be easy to explain how they all formed.

But that's why I went to the very early-universe evidence for dark matter. Because irrespective of what form ordinary matter took in the late universe, it was all the same in the hot early universe.

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panzuulor t1_j46p541 wrote

There isn’t enough ordinary matter in the universe to account for it all. All the exoplanets and non-radiating matter is only a tiny fraction of the matter we see. And that only accounts for a small part of the gravity we measure. That’s why dark matter is inferred.

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dastardly740 t1_j47nd3p wrote

To expand a bit, aside from the early universe point in terms of how we get anything smaller than a gas giant for dark matter. One thing that is possible even for hard to detect ordinary matter is that even non-detection sets limits for the size and quantity of those objects. So, surveys watching for occultations and gravitational lensing events on stars in Andromeda or distant galaxies or quasar by these objects if they made up the Milky Way's dark matter halo set limits on the size and quantity of planet sized objects and even planet mass primordial black holes. I believe the current boundaries for size and quantity of objects that have not been eliminated as significant dark matter contribitors is pretty small right now.

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Leemour t1_j484cvk wrote

Couldn't we just be wrong about some aspects of cosmology, something like relativistic gravity?

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Aseyhe t1_j48ddgg wrote

That is possible. However:

The early-universe evidence for dark matter is so strong that even the (non-mainstream) researchers who favor modifications to gravity to explain dark matter in galaxies still have to include dark matter in the early universe. Then they have to get rid of it by the time galaxies start to form to make way for their gravity theory.

Another perspective is that dark matter is a much simpler hypothesis than a modification to gravity. We already know of one dark matter species -- neutrinos -- and it's really not a stretch to suppose that there is another. The only special feature that we know it has to have is that it has to reach the observed abundance.

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forte2718 t1_j49mt2l wrote

I think XKCD said it best:

>Yes, everybody has already had the idea, "maybe there's no dark matter — gravity just works differently on large scales!" It sounds good but doesn't really fit the data.

The bottom line is that, even though we've explored quite a lot of modified-gravity / alternative-cosmology models, none of them have been able to fit all of the data even with some of the wildest and most contrived parameterizations. In basically every case, we can choose parameterizations that work to fit some datasets, but then those same parameterizations then go on to fail miserably at fitting other datasets ... and there just isn't any parameterization that works for them all at the same time, which means none of those models are actually viable as models of cosmology.

On the other hand, not only are dark matter models incredibly simple by comparison, but it is straightforward to parameterize them such that all of the datasets are well-fitted with the same parameterization, and getting to that point doesn't even require any specific model of dark matter — the generic idea of dark matter works so well that there are many different possible models of dark matter that work. It could be sterile neutrinos, or axions, or WIMPs, or light supersymmetric particles, the list goes on and on ...

When an idea works so well and with such simplicity and generality while every other idea explored falls flat on its face no matter how complicated you make it, you just have to take a step back and admit that maybe, just maybe, the simple and general idea is actually the correct one. It's kind of like ... if you're sitting there with pegs of different shapes and you find that the round peg fits the round hole but none of the other pegs fit it, well ...

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