Analogy is a slippery substitute for science. But to understand the dark matter problem we’ll try an analogy. Imagine you have a long string with a weight on the end. Now spin, with the weight pulling on the string as you turn in circles. If you spin fast enough, and the weight is heavy enough, the string will break and the weight will go flying off.1
Now let’s scale that up a little bit. Okay, quite a lot. The spinning is a whole galaxy. The weight on the string are stars at the outer edge of the galaxy. The string is the force of gravity. The dark matter problem is that the stars on the outer edges of galaxies spin much faster than they should. The visible matter in galaxies is insufficient to create a gravitational pull to hold those star in their orbits. They should fly off in to intergalactic space. The “string” isn’t strong enough for the “weights” to spin as fast as they do if it’s just visible matter that’s creating the gravity.
Either physics’ understanding of gravity is seriously wrong or there’s something – a whole lot of something – generating gravity besides the stuff we can see, visible matter.
In fact, the missing gravitational pull is so strong that visible matter can only be a small fraction of the universe’s mass. “Dark matter” is the astrophysics label for this unexplained effect. It’s not a description; it’s a placeholder name. It’s stuff we can’t see that seems to create gravitational effects.
We can’t see dark matter. That’s how it earned its name. But using Einsteinian gravitational lensing, it’s possible to map where dark matter might be, or at least where there is a gravitational lens effect with not visible matter. Which is kind of the definition of “dark matter”:
At first astrophysicists thought that the dark matter discrepancy might be due to MACHOs, MAssive Compact Halo Objects, like brown dwarf stars and black holes. But data from the EROS project in the late 1990s suggested that MACHOs weren’t numerous enough to account for all the mass of needed. In addition, other observations showed that the gravitational effects of dark matter occurred in places were MACHOs—which reside at the edges of galaxies—did not exist. Finally, black holes have a gravity signature that’s pretty distinctive, and those weren’t seen, either. So the “missing mass” likely wasn’t MACHOs.
Could it be WIMPs, which are Weakly Interacting Massive Particles? WIMPs would be a new particle, or family of particles, in the subatomic bestiary. Again, by definition, the only known way WIMPs interact with the world o visible matter is by gravity. But proposed theories for WIMPs suggest other possible, rare interactions. Two such experiments have been under way for a while now. So far, they haven’t turned up any clear evidence of WIMPs.
One involves filling a big vat with liquid Xenon, a noble gas element, and seeing if a WIMP collides with a Xenon atom. Some theories of WIMP predict collisions, which should give off a photon, a flash of light, that might be detected. No luck so far.
Another theory predicts that WIMPs should collide and decay. If the theory is right, then we might be able to see evidence in denser regions of dark matter, where collisions might produce an excess of energetic particles like positrons, the antimatter partners of electrons. There are hints, but it’s far from conclusive. Astrophysicists have found excess energetic particles in data from the Russian-European PAMELA satellite in 2008, and again in data from the Alpha Magnetic Spectrometer on the International Space Station in 2013. However, neither data set can determine if the excess was indeed the result of dark matter particle collisions or if its from exotic sources of visible matter like pulsars (a type of neutron star).
There are other, wilder theories, and experiments trying to confirm them.2 But the central fact to this point is that dark matter, like dark energy, remains a completely unresolved puzzle. Science can map its effects, but not what it is. More than a quarter of the universe, pretty much unknown. And another two-thirds of the universe, dark energy, is even less well understood than dark matter.
When you can’t detect 95% of the universe, when you can’t confirm by observation that missing 95%, it suggests that current theories astrophysics may be headed into a scientific revolution, a Kuhnian shakeup. That would be cool.