> you could argue the same about dark matter/energy.
Well, not really. The physical justification is, the multitude of observations that appear consistent with the theory that there is some form of matter out in space that does not interact with photons. It's not something someone just had a dream about one night - it's a real-world observation, with telescopes.
If, tomorrow, a laboratory discovers a new or existing particle that is able to actually, or even just apparently, avoid interacting with photons and matter, then dark matter could be done and proven.
By contrast, most dark matter competitors require us to rethink large swathes of what we know about physics and cosmology. That doesn't mean they're necessarily wrong, it just means the bar of skepticism is a bit higher.
> Well, not really. The physical justification is, the multitude of observations that appear consistent with the theory that there is some form of matter out in space that does not interact with photons. It's not something someone just had a dream about one night - it's a real-world observation, with telescopes.
This is it. Everything points towards it being some kind of invisible matter. Scientists know that the very idea is preposterous!
But you can't make up some 'cleaner' hypothesis without also holding that idea up to the same standards as Cold Dark Matter, and to date they have tended not to.
This hypothesis is based on the idea that the physics isn't uniform throughout the universe, a philosophical idea that has been debated for over a century and so far, all the evidence points to physics being uniform throughout the universe.
Claiming physics isn't uniform throughout the universe is more problematic than positing the existence of dark matter/dark energy. Lacking any evidence for either position, the principle of Occam's Razor informs us to stick with dark matter/dark energy.
One of the purposes of exercises such as this is to get people thinking of new ways to falsify existing models. So far, dark matter/dark energy are in remarkable alignment with observations. Falsifying either of these ideas would be huge.
Dark energy is a label for "whatever it is that causes the expansion of the universe which we have hard observations of."
Dark matter is a label for "whatever it is that causes the extra gravitation observed across multiple scales in the universe without also causing other indications of matter interaction like photon emission."
These are easy-to-hold handles for the collection of observations for phenomena we know to exist. We don't know the cause, but we know these things are occurring in the universe because we have demonstrated their existence through repeated observation.
I get your point, but "So far, dark matter/dark energy are in remarkable alignment with observations." is not in accord for "whatever it is that causes... <our observations>". It would be a tautology at that level, equivalent to saying "our observations are in accord with our observations".
Dark matter/dark energy work in the existing framework of GR and align well with observations. That's why they're so heavily favored. It's simple.
Saying the laws of physics may be different in different parts of the universe is a statement that so far can't be substantiated, and we've been trying for over a hundred years now, is far more problematic. That's a dire option that we don't desire to pursue unless we're absolutely forced to - and we're not being forced to.
Between the two options we're going to stick with dark matter/dark energy until evidence suggests otherwise, or until a better, and simpler, idea comes along.
I've always found "Dark Matter" explanations unsatisfying. No-matter how deep you dig you always end up at a point where dark matter is nothing more than a contrivance.
Nothing from the observable universe, just bundling all the mathematical loose ends together and stashing them under the rug.
It's actually the opposite. It's observed by the effect it has on gravity in the region. It's explicitly not observed by blocking or emitting photons, but that's only one way of observing things.
Dark matter's effect is only needed because there are other factors that are presupposed. If you set the age of the universe at 13.787 Billion years old (oh wait, now they think it's 27.3 Billion now), then the large scale structures we see in the universe make no sense.
So to my point, we are not "seeing" dark matter anywhere at all... What we have is a failure mode in our current models to explain the phenomena we see. And to fix it they plug in magic invisible heavy stuff. Literately a chunk of math to balance equations in the rotations of galaxies.
Yes, cold dark matter is something conjectured to explain what we see, because we see these irregularities and we need something to explain what is causing them. If we weren't seeing those irregularities we wouldn't need it.
>> What we have is a failure mode in our current models to explain the phenomena we see.
The thing is, you don't know that, and that concept has already been thought about many times over.
What is surprising about CDM in particular is that it is ridiculous and yet it survives observation and theory better than every other hypothesis to date. It is more intuitive that the mathematical equations are wrong, or they've measured something (like the age of the universe) wrong, or there's some kind of effect that hasn't been accounted for. Yet after testing all that, CDM fails to be thoroughly disproved. The vast majority of evidence points to it being a real thing. If it looks like a duck and it quacks like a duck...
I don't think it's appreciated enough that astrophysicts do know that this is weird. That's why it's so interesting.
However, you need to come up with a competing theory that actually holds up to rigor. It's no good saying "it must be this" without any supporting evidence to support it. It's simply a guess, sci-fi, not science.
Yes, it's entirely possible that it will be explained by some part of physics we currently have no knowledge of. Some addition to the standard model, perhaps, or some miniscule effect that falls out of quantum gravity, whatever that may be. But those things aren't even a hypothesis at this point, just pure speculation without justification.
you could argue the same about dark matter/energy.
> It cannot be tested or falsified by current or near-future experiments.
Did we actually directly observe dark matter or dark energy now?