Dark Matter is Not "Bunk Science" -- But it has problems

[This is a transcript.]

Physicists say that 80 percent of the mass in the universe is dark matter, and not only this, dark matter is supposedly all around us, we just can’t see it. Sounds pretty crazy, doesn’t it. Indeed, every time I mention dark matter, I get flooded with comments about how silly the idea is. “Fantasy folly.” “bunk science,” “epicycles,” “a mirage”, “unscientific,” and on and on it goes.

I really feel for astrophysicists, but those complaints are not totally wrong. Let’s have a look.

We have two separate theories that together explain how the universe works. One is the standard model of particle physics. It contains all the matter and radiation and explains how that stuff interacts. The other is Einstein’s General Relativity, yes that guy again. His theory tells us how space-time curves in response to matter, and how the matter moves in the space-time.

The issue is now that if you use these two theories to try and describe what we observe out there in the cosmos, it just doesn’t work.

Astrophysicists have collected lots of evidence that contradicts the predictions of those two theories combined. Galaxies rotate too fast, galaxies in galaxy clusters move too fast, there are too many gravitational lenses and they’re too strong, galactic filaments seem to have too many structures, and more. It’s a really long list. And we can’t chalk this up to the missing unification of gravity and quantum theory. Because while we don’t yet have that theory, we can estimate when its effects would become important, and it’s not for things as large as galaxies.

But physicists figured out that they can explain all that data quite simply, by introducing a new kind of matter. This is the so-called dark matter. It’s stuff that we can’t see directly, we can only infer its presence indirectly from its gravitational pull. Is like you know there’s chocolate cake in the fridge even though you can’t see it.

This may sound like a cheat, because it seems that this doesn’t explain anything, but when it comes to explanations in science you always have to ask how much data you can explain with how many assumptions. And on that count, the original idea of dark matter does very well.

It’s because while “matter” sounds quite vague, in physics it means something very specific. The word tells you how the stuff behaves. And the original idea was that you’d really only need to know how much there is of this stuff in the entire universe. Because once you know that, then how it’s distributed should come out of Einstein’s equations.

And that’s why dark matter is a good scientific theory, in principle, because it explains a lot from very little. All you need to know is this average density, and the rest should come out automatically.

In principle. In practice what’s happened is this. Telescopes became better so data became more precise, and computer simulations became better, so predictions became more precise. And over the course of time, that simple dark matter story didn’t fit with the data all that well anymore. Physicists then began to make the story more complicated.

For example, in principle, the idea of dark matter should tell you how the stuff is distributed inside of galaxies. But the results of computer simulations for this didn’t fit with observations. So then astrophysicists just assume a distribution that fits the data, and they say that this comes about by some complicated and not well understood physical processes: supernovae, black hole jets, non-linear feedbacks, flying pigs, and so on. And that’s where things get somewhat fishy because each time you adjust the theory to make it fit the data it loses some of its explanatory power.

It didn’t help that particle physicists added further superfluous details that weren’t needed to explain the data, such as exactly what kind of particle it is, or how it interacts, and how it supposedly killed the dinosaurs and so on. I wish that was a joke, but it isn’t.

And that’s why the dark matter theory has become so controversial. Because for all practical purposes it’s become impossible to falsify.

Indeed you might have followed an example for this on this YouTube channel. When the Webb telescope had just launched, I made a video about what it might see. I explained that it’d see very young galaxies, and that dark matter predicts that they should form gradually and slowly. The competing theory, modified gravity, in contrast, predicts that galaxies should start to form fast and early. By now it’s clear that the data actually agree better with modified gravity. But what’s happened is just that astrophysicists have tried to come up with an explanation for how dark matter could work differently than they predicted.

In summary, dark matter, in its original version was a good scientific theory. It was a simple explanation for a lot of data. But in the past decades it’s become increasingly more complicated to make it fit the data and that has much decreased its explanatory power. It’s certainly not “bunk science”, but the idea has problems. Is it right or wrong? I hope we’ll find an answer to that in my lifetime. Though I suppose that just makes a lot of astrophysicists wish I’ll die soon.

Thanks for watching, see you tomorrow.