By Don Lincoln, Fermilab
No matter how persuasive different data appears to point in the direction of there existing unseen matter, we still don’t know what it is. Over the past half a century, astronomers have made many observations that clearly rule out a vast swathe of possible proposals of what dark matter is. So, if I can’t tell you what dark matter is, at least let me tell you what it isn’t.
Suspect Number One
The most obvious possible form of dark matter isn’t anything exotic. Instead, it’s what is called baryonic dark matter, which just means that it is dark matter made of the familiar baryons, like protons and neutrons. There are a couple of clear options. The first is just that there exists a whole bunch of hydrogen gas out there, cold and invisible, but enormously massive.
And that was a possibility in the early days when dark matter was still a new idea and optical telescopes were all the rage. Astronomers can now use giant radio telescopes to snoop on lots of things that don’t emit visible light. One of those things is the radio emission of hydrogen gas.
And there’s a lot of gas out there. Over the entire universe, there is something like 10 times more mass in hydrogen gas than in all of the stars and galaxies. Hanging some numbers on that, the glowing things in galaxies make up about half a percent of the energy and matter of the universe. The bottom line is that we know about cool interstellar gas and it’s not dark matter.
Heavy but Invisible
The second obvious form of baryonic dark matter is what is called Massive Compact Halo Objects, or MACHOs for short. MACHOs are anything that are heavy and compact and invisible. These could be black holes or brown dwarfs. These objects could wander the cosmos, essentially invisibly.
Then there’s the possibility of rogue planets, which are planets that have been thrown out of their solar system and now wander the galaxy, unattached to any star. You’d think that these objects would be hard to see, but astronomers are clever people. They realized that Einstein’s theory of general relativity would be helpful.
Einstein realized that this behavior had an interesting corollary. Suppose you were looking at a distant star and another star passed directly in front of the distant one. The gravitational field of the near star would bend the light from the distant star as it crossed the line of sight. The result would be that the distant star would appear as a ring or a halo around the nearby star. This is an example of gravitational lensing and the phenomenon is called an Einstein ring.
However, the same phenomenon can be used to search for MACHOs. If you look at a distant star and a heavy and invisible massive object passes in front of it, the distant star will appear to brighten temporarily before returning to the original brightness as the intervening star moves on. This is called microlensing.
In any event, during the 1990s, astronomers looked for examples of microlensing. And they did see examples of microlensing with the right optical characteristics. When they analyzed their data, they found that there just aren’t enough MACHOs out there to explain dark matter.
This article comes directly from content in the video series The Evidence for Modern Physics: How We Know What We Know. Watch it now, on Wondrium.
Something Never Seen Before
If we’re sure that dark matter isn’t ordinary matter, then we need to consider other options, which is to say types of matter that nobody has ever seen before. So, what might it be? Well, there have been lots of proposed new ideas, many of them highly specific.
However, there is a generic term that was popular for many years. In contrast to MACHOs, this exotic form of dark matter is called a WIMP, short for weakly interacting massive particles. WIMPs are not well defined. If they exist, they are particles, kind of like stable neutrons. These particles would be pretty heavy, ranging from the mass of a neutron up to thousands of times heavier than neutrons. That’s the massive particle bit.
They would also interact weakly, meaning through the weak nuclear force. We now know that this isn’t true exactly, and the meaning has morphed from weakly meaning interacting via the weak nuclear force into the more generic meaning of the term, which is just that the interaction is very weak, perhaps weaker than the weak force and perhaps due to an unknown weaker force.
How Can We Detect Dark Matter?
So, what experiments have been done to narrow down the properties that dark matter might have? Well, there are three experimental approaches that physicists have used to search for dark matter. They all assume that dark matter interacts with ordinary matter.
The first is that these particles might bump into ordinary matter particles, much like an electron might bump into another electron. That’s one possible interaction. If we found dark matter this way, it would be called direct detection, because we’d see the effect of a dark matter particle directly hitting a matter particle.
Another possibility is that if dark matter exists and it’s basically a particle gas that permeates the galaxy, maybe there are both dark matter and dark antimatter particles. If they exist, then possibly a dark matter and dark antimatter particle might bump into one another and annihilate. Possibly what would come out of that interaction would be matter. If we found dark matter this way, this would be called an indirect detection.
Then there’s the third way that one might imagine detecting dark matter and that’s creating it out of energy. Using large particle accelerators, we collide together subatomic particles and use that energy to create particles that didn’t exist before the collision. This particular method of searching for dark matter is especially attractive, as the creation and detection and the entire process can be designed and studied very carefully.
Common Questions about Theories about the Nature of Dark Matter
There are three possible forms proposed for dark matter: baryonic dark matter, Massive Compact Halo Objects (MACHOs), and Weakly Interacting Massive Particles (WIMPs).
MACHO, which stands for Massive Compact Halo Object, is a proposed form of baryonic unseen matter. These are halo objects that could be anything compact, heavy, and invisible. They could be brown dwarfs, black holes or even rogue planets wandering around the cosmos.
Dark matter can be created out of energy using huge particle accelerators, in which subatomic particles are collided, generating energy that is used to create brand-new particles.