Dark Matter

What is it? How do we know? Oh, and a brief meditation on God.

A warning: I’m not an astronomer or physicist and this is massively outside my area of expertise. Still, it’s cool and I’ve done some reading about it, so I thought I’d write about it, but it’s definitely possible that I’m getting stuff wrong!

Imagine if historians discovered a vast invisible civilization. Through gathering data about heat signatures or something, they discovered that there were lots of invisible people that left behind no archeological remains or documents, that didn’t even give off light. If you’d come across their civilization, you’d simply walk right through it, never aware that you had interacted with it. Despite the invisibility of the billions of invisible people, we’d come to know them through their effects.

This would be a staggering discovery. It would be a discovery like nothing historians have seen before, one that puts even the grandest of historical projects to aim. Discovering that Atlantis was real would be nothing compared to this discovery. It would be perhaps the most substantial discovery ever made.

This is pretty much what physicists have done with dark matter.

Dark matter is a kind of invisible matter that pervades the universe. It’s pictured below:

Or consider the following photo, which shows what it would look like if there was a bunch of dark matter in front of an elephant:

It doesn’t give off light or electromagnetic radiation. The visible matter that we observe makes up only around 5% of the total matter, with the other 95% being dark matter (26%) and dark energy (69%). This would be a bit like discovering that 95% of the food in your refrigerator was secret, invisible food that you could only discern through detecting its gravitational effects.

That dark matter exists has become the consensus view among physicists. But what caused physicists to accept such a strange and specific theory (my information here comes from the book Space at the Speed of Light, by Becky Smethurst, an astrophysicist at Oxford who also has an excellent YouTube channel that helpfully explains new discoveries in physics). In short, the answer is that the universe really behaves like there’s dark matter. While we can’t observe dark matter directly, we can come to infer that it exists by its effects—by noticing that the way things behave is exactly what we’d expect if there was invisible matter filling galaxies.

The first line of evidence involves galactic rotation curves and was originally uncovered by astronomer Vera Rubin. In our solar system, stuff nearer to the sun travels much faster. This is straightforward enough—almost all the mass in the solar system is occupied by the sun, so the stuff nearer to it moves much more quickly. Galaxies should work the same way. Because most of the stars and planets in galaxies are in the center, we’d expect the stuff in the center of galaxies to rotate much more quickly. But that’s not what we observe.

What we observe is that the outer bits of galaxies rotate at the same speeds as the center of galaxies. This makes a lot of sense if the mass is mostly not in the center of galaxies. But because most of the mass we can see is in the center of galaxies, either something about our understanding of physics is off or there’s a lot of invisible matter in the outer edges of galaxies. While this alone is not enough to prove dark matter, it’s certainly some significant evidence, because it’s a feature of the world that makes more sense—that’s rendered more probable—on the assumption that dark matter is real than that it’s not.

The second piece of evidence comes from gravitational lensing. Light is affected by gravity, which is why, for instance, black holes don’t give off light. They suck in light with such great force that none exits them. But crucially, gravity also bends light, changing its trajectory.

If there’s an object that gives off light, astronomers can measure how much the light is bent. If a Quasar, for instance, gives off light that gets bent by a galaxy, scientists can measure the degree to which the light bends and see how much mass is in the galaxy. The image below can help illustrate this phenomenon:

But what scientists have found is that the amount of mass measured by lensing is more than the mass we observe in galaxies. If all the mass were from gas, planets, stars, and the like, the gravitational lensing would be much weaker than it actually ends up being. This is once again very naturally explained on the assumption that there’s loads of invisible matter, but makes very little sense on the alternative assumption. Scientists have also looked at whether this could be explained by black holes and neutron stars, but found that there would have to be so many to explain the data that this would have other observational detectable consequences that we don’t see. If there were a lot of black holes, for instance, we’d see the black holes exert other lensing effects on a smaller scale.

Recently, when two galaxies collided—together called the bullet cluster—scientists were able to analyze gravitational lensing to find what parts of the galaxies cause the lensing. They found it was mostly the outer bits that didn’t release light. This means that there’s a lot of dark matter—most of the mass in those galaxies comes from the dark matter outside of it.

Third—and this bit of evidence was discovered fairly early, but made precise more recently—we can look at different ways of measuring galactic mass. The first way is to just see how bright a galaxy is, which would work fine if all the mass was visible. The second way involves calculating the velocity of galaxies—I don’t quite get how this works, but the important bit is that it’s not just detecting the observable matter. When we do this, we find that if you measure brightness, you get much lower measurements for mass than if you measure velocity to determine mass. This means that a lot of the mass comes from stuff that doesn’t make galaxies brighter.

Fourth—and here I’m just going from Wikipedia—there’s something called the Virial theorem. It’s a formula that, if I’m understanding correctly, tells how particles will move around when affected by a force like gravity. Apparently the way stuff would be expected to move according to the Virial formula doesn’t match how they actually move, unless we add in the assumption that there’s a ton of dark matter.

Fifth, galaxy clusters’ behavior is best explained by the assumption that there’s dark matter. Galaxy clusters involve lots of different galaxies bound together and interacting. It turns out that many of them would have fly apart if there was only as much matter as we can observe, proving there must be some invisible matter.

There’s a lot more evidence, but that was about as much as I could get my head around without my brain breaking. The other evidence on Wikipedia was described like this:

In astronomical spectroscopy, the Lyman-alpha forest is the sum of the absorption lines arising from the Lyman-alpha transition of neutral hydrogen in the spectra of distant galaxies and quasars. Lyman-alpha forest observations can also constrain cosmological models.^[82] These constraints agree with those obtained from WMAP data.

Umm…yeah!

There are other theories. The most popular competitor is MOND, according to which the theory of gravity needs to be modified to account for these anomalies. But these are pretty widely rejected, in part because they have to be really wonky and ad hoc to explain all the many things that dark matter explains.

Now, there are all sorts of theories about what dark matter is. Ultimately, we don’t really know, and it remains a pretty massive mystery. But there are lots of different theories, and hopefully one of these days we’ll definitely find the answer!

Dark matter is interesting because it illustrates the tremendous power of science. We evolved on a massive rock and evolved to eat berries and reproduce. Yet we’ve been able to discover that almost all of the matter in the universe is invisible dark matter that we can’t see. That’s pretty fricking cool!

It also illustrates how strange the universe is. Almost all of it is unknown, hidden, dark—exerting gravity but unable to be touched or seen. As I said at the start of this article, it’s a discovery on the order of finding out that most historical civilizations were invisible and only able to be detected by seeing their heat signature.

Now, I hope you’ll pardon theological reflection in a science article, but I also think dark matter is a pretty good analogy for God.

We don’t see dark matter directly. Rather, we see that there are a bunch of surprising and improbable things that happen. We come to realize that those things are explained by the hypothesis that there’s invisible matter—matter like the matter in our experience in some ways, different in others. Because the notion that there’s invisible matter that exerts gravity makes so much sense of otherwise surprising facts, even though it’s weird, we conclude that there really is that matter. It’s a bit like the kinds of matter we see, but different in other ways.

When it comes to God, we observe lots of surprising things. Constants of the universe finely-tuned—falling in an extremely narrow range needed for the formation of complex structures; complex laws that produce valuable states of affairs; consciousness that fits poorly into our picture of reality, surprisingly harmonious. That’s all super surprising!

So we posit something a bit like the stuff in our experience to explain it. Because all these things are very surprising and very valuable, we posit that there’s a good agent to explain it. Unlike the agents in our experience, if this agent is to explain the rest of reality, it must be simple and powerful. We posit something very simple—slightly alien, slightly weird, but somewhat similar to the things in our experiences. We do this because it explains the world so well.

Like dark matter, if God just explained one or two things, we could write those off as anomalies. But when there are nearly a dozen otherwise bizarre anomalies nicely explained by the theory, whether it’s that God exists or the dark matter does, we say that the theory is true. When gravitational lensing, galactic mass, the arms of spiral galaxies, and whatever the heck a Lyman-alpha forest is all attest to there being a bunch of extra invisible mass, it’s reasonable to think that, weird as it is, there really is invisible matter pervading the universe. When atheism is filled with anomalies—the surprising existence of a physical universe, laws, nomological harmony, fine-tuning both a priori and from physics, consciousness, and dozens more things—it makes sense to believe in God.

Dark matter isn’t primarily believed in because it makes advanced predictions but because it explains otherwise anomalous data. Nothing about galactic mass makes sense except in light of dark matter. Similarly, nothing about the universe makes sense in except in light of God. So many improbable things happened that were needed to produce agents that the world makes much more sense on the assumption that it’s being steered to produce agents. God and dark matter are both invisible, but reasonable to believe in, because, though we don’t see them, we can observe their effects, and the only comprehensive way to make sense of reality must invoke them.

This is really going to bite me in the rear if two decades from now physicists all agree that MOND is right, and dark matter is universally rejected! Hoping that doesn’t happen!