When Not Seeing Leads to Believing

Humans have always searched for explanations for the unknown. When we can't see a rational cause for something, we inevitably conclude that there's some kind of mystical, supernatural force at work.

Even Albert Einstein did this, in a manner of speaking, when he added the cosmological constant to his theory of general relativity in order to achieve a static universe, which was the accepted theory at the time. Einstein later called this his "greatest blunder."

More recently astrophysicists came up with something called dark matter to explain the "missing mass problem": astronomers cannot find enough mass with telescopes to account for the gravitational effects they observe in the galaxies around us.

Galaxy and its halo

By the 1930s astronomers had found that nearby galaxies were rotating faster than could be explained by the estimates of the masses of their visible components (stars and gas clouds): there had to be some kind of invisible matter providing most of the mass that held these galaxies together. Even accounting for the black holes that we know are at the center of most galaxies, there still wasn't enough mass.

Current theory postulates that most dark matter is some kind of special "nonbaryonic" matter, hypothetical axion particles completely unlike mundane protons, neutrons and electrons. However, the theory does grant that a small portion of the missing mass is regular "baryonic" matter, residing in massive compact halo objects.

They also came up with something called dark energy to explain why the universe keeps expanding faster and faster. Dark energy is sort of like antigravity, an idea that raises a lot of hackles. One current theory goes into great detail, calculating that the universe is composed of 4.9% regular matter, 26.8% dark matter and 68.4% dark energy.

This is the mirror image of seeing is believing: not seeing mundane physical matter led scientists to believe in the existence of strange and esoteric dark matter.

I admit to being skeptical about dark matter (and dark energy). It smacks of the sort of mystical answer that I distrust: we can't see the missing mass, so that must mean there's something special and weird going on. I've always thought that the simpler Occam explanation is that we just can't see the missing mass because it's dark out there. Dim red dwarf stars, brown dwarfs (starlike objects too small to emit visible light) and cold dust clouds are essentially invisible to our telescopes, or the masses of galactic core black holes could be underestimated, or there could be smaller undetected "loner" black holes orbiting in the galactic periphery.

Well, now it turns out that someone may have found that missing mass, in exactly the place we should have expected it. This discovery has the potential to completely upend decades of theoretical astrophysics.

Using the Hubble Space Telescope, Jessica Werk and her team at the University of California, Santa Cruz, used light from quasars to detect haloes of cold gas around galaxies ("cold" is relative: the gas is at 10,000 degrees Celsius). The gas previously observed in galactic haloes is about 1 million degrees -- at that temperature the gas emits photons and can be detected by our optical and radio telescopes.

The cold gas clouds absorb some of the quasar's light as it passes through, allowing Werk's team to detect traces of carbon, silicon magnesium and hydrogen. They calculated that there may be 10 to 100 times the amount of cold gas than astronomers previously thought existed, potentially making up all of the missing mass.

If these observations hold up and the cold gas haloes do account for all the missing mass, that doesn't mean the scientists who theorized about dark matter were wrong to do so. They were working with the best data they had, and some aspects of the theory could still be true. And this finding still doesn't explain why the universe's expansion seems to be accelerating (though that could be another observational inadequacy).

The biggest mistake we can make in science is assuming that our observations are complete, that the beliefs we have now are final and can't possibly be changed. Even with the best tools and techniques at their disposal, scientists could not detect the missing mass. So, rather than just assume it was there -- which would definitely have been wrong -- scientists sought out other explanations. And those explanations led them into really esoteric places.

Following that path wasn't wrong: it's what scientists are supposed to do. But once we have the new data, and we have reverified that data several times to ensure that we aren't being misled this time too, we have to go back and revisit and revise everything, and chuck out the theories that don't support the facts.

That's the process of science: going back, testing our assumptions, making the same observations again and again, in new and different ways and from different directions. Making sure that we get the same results, or if we don't get the same results, understanding why we didn't, maybe correcting our experimental methods, or possibly stumbling upon another secret of the universe.

When we do, we often find we no longer need supernatural explanations to explain what we see -- or don't see.