What is dark matter?
Many astronomers will tell you that it’s a mysterious substance, as yet unknown to physics. It has many strange properties, including invisibility. And the Universe contains a lot of it: there’s four to five times as much dark matter as there is ordinary matter (the stuff that makes up stars, planets, and so on). In other words, for every galaxy you see, there’s four or five galaxy’s worth of stuff that you don’t see.
But other scientists disagree with this.
Some will tell you that dark matter doesn’t really exist. Instead, the observations that are claimed to show the existence of dark matter are better explained in other ways.
What’s going on here?
I discussed dark matter briefly in Volume II and again in Volume III. Although I mentioned that there were alternate explanations, I didn’t have time then to discuss them. So let’s do that now.
Why Many Astronomers Believe in Dark Matter
There are places in the cosmos where we observe things moving more quickly than perhaps they should.
For example, a typical galaxy contains about 100 billion stars. These stars are all orbiting their common center of mass—basically, the center of the galaxy. We can use celestial mechanics (the physics of motion for astronomical objects) to calculate the velocities of stars within the galaxies. A graph of their expected speeds looks like this green curve:
But when we measure the stars’ actual speeds, they don’t match this graph. The inner stars are moving as we expect, but the outermost ones are moving more quickly, as shown in the orange curve:
By the way, this is called a galaxy rotation curve: a graph of the orbital speeds of stars, versus their distance from the galaxy’s center. And it turns out that ‘discrepant’ curves are common.
Many galaxies have this divergence between the outer stars’ expected speeds and their observed speeds.
So what’s going on here? One possible explanation is that there is a lot of matter—a lot of physical stuff—arranged in a “halo” (a spherical shape surrounding these galaxies).
If there were enough matter in a halo, then its mass would gravitationally affect the galaxy’s stars, and this would alter their orbits into the patterns that we observe.
(The graphic above shows an artist’s conception of such a halo around our Milky Way galaxy.)
However, this would require a lot of mass—several times the amount of mass that the galaxy itself contains. And even after exhaustive searching for halos around our galaxy and others, we don’t see anything remotely close to the amount of mass required.
So, it appears that there is a lot of mass that’s missing. In fact, this used to be called the missing mass problem.
Today, it’s usually not called that anymore, because most astronomers say that the mass isn’t really missing. They say that there is indeed a lot of matter surrounding these galaxies.
We’re just unable to see it.
This has become known as dark matter. It’s physical stuff, thus the word “matter”—but we don’t see it, thus the word “dark.”
Among astronomers, dark matter is a frequent topic of conversation. In addition to the unexpected speeds of stars within galaxies, there’s a similar issue where many galaxies are moving with unexpectedly high speeds within their galaxy clusters.
Yet another issue is found in gravitational lensing, where matter gravitationally distorts our view of far-away objects. There are some examples where we observe lensing, from which we can calculate the mass that is producing it. But the mass that’s calculated is more than the mass we see.
Again, this “missing mass” is attributed to dark matter.
Figuring out the identity of this stuff is a major area of research within astronomy today. There are several possibilities for what dark matter could be:
- Ordinary matter made of atoms, in a form that’s hard to see (such neutron stars, small black holes, very dim objects, etc.)
- Neutrinos (tiny electrically-neutral particles, much smaller than atoms).
- Some new form of non-atomic matter that’s unknown to physics.
- A mistaken idea, because dark matter doesn’t actually exist. The discrepant speeds of stars, galaxies, etc. have other explanations.
Let’s discuss each of these possibilities.
Could Dark Matter Be Ordinary, Atomic Matter?
Early on, many astronomers thought that dark matter could be ordinary matter, made of atoms. But it was in a form that is difficult to detect, such as:
- Neutron stars
- Small black holes
- Brown dwarfs
- Dim white dwarfs
These proposed objects became known as MACHOs (Massive Compact Halo Objects). Multiple research projects were launched to find them.
For several years, millions of stars were observed for microlensing events—where a MACHO would pass in between us and a faraway star, gravitationally distorting its image for a short time. This would allow us to detect that MACHO even while being unable to see it directly.
Some microlensing events were indeed observed. But these fell short of expectations.
So, although it seems that there might be some MACHOs out there, they are not enough to fully explain the missing mass.
As a result, few people are still pursuing MACHOs today as a viable candidate for dark matter. The astronomical community has, for the most part, abandoned this idea.
Could Dark Matter Be Neutrinos?
Another possibility for dark matter is that it’s made up of neutrinos.
Neutrinos were first hypothesized in 1930, but it took 26 years to physically confirm their existence. By their nature, they are extremely difficult to detect.
These particles don’t interact with electromagnetic radiation (visible light, radio waves, x-rays, and so on).
This makes them very challenging to detect, since most of our scientific instruments (telescopes, radio telescopes, etc.) are based on electromagnetic radiation in one form or another.
It also means that neutrinos would seem to be an excellent explanation for dark matter. A halo of neutrinos would have mass, but would not be visible.
Nevertheless, neutrinos are not very popular as candidates for dark matter. If they were abundant enough to be the source of the missing mass, they would add serious problems to the Big Bang model (in addition to the numerous problems it already has), especially in galaxy formation.
So, neutrinos as dark matter are compatible with the observations, but incompatible with the Big Bang model. Thus, most secular astronomers reject them as the explanation for dark matter.
Could Dark Matter Be a New Form of Matter?
Now let’s discuss the explanation endorsed by most secular astronomers today.
Since dark matter must exist, but we don’t see it, and it can’t be neutrinos, it must be some other form of matter that can’t be seen.
Like neutrinos, it wouldn’t merely be difficult to see. Like neutrinos, it must be impossible to see. Like neutrinos, visible light and all other forms of electromagnetic radiation must pass right through it.
But unlike neutrinos, these mysterious particles would be unknown to physics.
These mysterious particles have become known as WIMPs (for Weakly Interactive Massive Particles. And yes, there’s some tongue-in-cheek going on here with the names: MACHOs versus WIMPs).
The problem with WIMPs is that they don’t fit within the standard model of particle physics. They aren’t merely unknown to physics—they’re outside of it.
However, there is a proposed extension to the standard model, known as supersymmetry. WIMPs would fit within it. And so, for the last several years, collider experiments have been running, trying to find some evidence for supersymmetry.
So far, all these experiments have failed. It appears that supersymmetry has been falsified.
This leaves no room within known physics for WIMPs. Thus, there’s no theoretical reason to expect them to exist.
But many secular astronomers have not been deterred by this. There are numerous years-long experiments that are trying to observe WIMPs anyway.
So far, all of these experiments have failed too.
Despite the lack of theoretical support for WIMPs, and the lack of evidence for them, and the growing amount of evidence against them, most of the secular astronomical community still clings to this idea.
Why? Because this issue is not just about the possible discovery of some new particles.
Quite some time ago, when WIMPs were first proposed, it was quickly realized that they would be a dandy way to solve certain problems within the Big Bang model.
(Specifically, some problems with the formation of large-scale structures early in the Universe’s history.)
If WIMPs do not exist, then the Big Bang model will have even more serious problems explaining cosmic structures than it already has.
And so, many secular astronomers are continuing to search for WIMPs, despite the growing amount of evidence against this idea. Their adherence to the Big Bang model is overriding other factors.
However, there are still a few secular astronomers who do not believe in WIMPs. This brings us to our next possible explanation of the missing mass.
Does Dark Matter Even Exist?
Perhaps there’s a better explanation for the discrepant observations—an explanation that doesn’t require a whole new class of invisible particles, outside of known physics.
In fact, this explanation doesn’t require any unseen matter at all. Therefore, dark matter wouldn’t exist.
Earlier I said that the evidence for dark matter comes from discrepant galaxy rotation curves, the speeds of galaxies within galaxy clusters, and a few other things.
But maybe these observations aren’t discrepant at all. Maybe the problem is that we don’t correctly understand celestial mechanics at large scales.
Maybe a correct understanding of celestial mechanics would show that these objects should move exactly as we see them doing.
But celestial mechanics is a fairly straightforward application of our understanding of gravity. So, if we’re incorrect about celestial mechanics, then we also don’t understand gravity correctly, at least on ultralarge scales.
There have been several scientists who have suggested exactly this. And they have proposed different ways in which our understanding might be incorrect, and how we can fix it.
It’s outside the scope of this article to explain all the various “modified gravity” theories. It’s interesting though that modified gravity can seem to explain some (although not necessarily all) of the ‘discrepant’ observations better than the ideas that require matter.
(I should also mention that many of the arguments against these theories are often founded on Big Bang assumptions. For example, the Cosmic Microwave Background radiation is interpreted as evidence for the Big Bang. As such, it’s then judged to be inconsistent with some of the modified gravity proposals. Certainly, if the Big Bang model were true, then this would be a valid argument against modified gravity. But since the Big Bang model isn’t true, this approach is invalid.)
Another Possible Explanation
Every ‘missing mass’ solution described above assumes that we have to explain how stars can be moving so quickly within galaxies, while still remaining in stable orbits within the galaxies—or how galaxies can be moving so quickly, while still remaining in their clusters—and so on.
But there’s one last possibility that few people mention. What if the stars aren’t actually in stable orbits within their galaxies? What if over time, galaxies are dissolving?
Similarly, what if the galaxies themselves aren’t actually gravitationally bound to stay within their clusters? And so on.
This explanation would certainly fit the observations. But it’s never mentioned, at least not within the secular astronomical community, because it requires all these objects to be very young—not billions of years old.
In fact, even many creationists are uncomfortable with this idea. As an astronomer friend of mine asked, “Why would God create these things in an unstable configuration?”
Well, maybe He didn’t. Maybe this was one of the results of the Fall. (After all, the Bible says the whole creation is groaning as a result of man’s sin.)
Or maybe He did. Maybe we’re making unfounded assumptions about how He would create things. After all, the Bible doesn’t speak about this issue one way or the other.
Our expectations about stability are ultimately based on how we think God should have done it. But He’s sovereign, and made the Universe according to His will—not ours.
Note that I’m not saying this is, or isn’t, the correct solution to the dark matter problem. I’m merely pointing out that most people are making an assumption about these objects, often without realizing it.
And it’s always helpful to be clear about our assumptions.
Which proposal is correct? What is dark matter really?
Obviously, at this point, nobody knows for sure.
Some of it could be made of MACHOs. Perhaps the majority of it is made of neutrinos.
Or perhaps one of the modified gravity theories is correct. I’m personally rather intrigued by some of these proposals—they would require Einstein’s relativity to be a subset of a larger and different understanding of spacetime, which would be exciting.
To me, the least useful theory is one that requires a new invisible particle—or a whole new class of invisible particles—all of which are completely outside of known physics, and against which there is a growing body of evidence.
Nevertheless, that’s the preferred theory in the secular astronomical community today.
Such are the fruits of a commitment to the Big Bang model.
Image credits: ESO/L. Calçada [Artist’s impression of Milky Way]; NASA/JPL-Caltech [Ultraviolet image of Andromeda galaxy in rotation curve graphics]