A mile underground, in a converted mine somewhere in South Dakota, scientists have been trying to detect an elusive substance that makes up around 27 per cent of all the mass and energy in the observable universe: dark matter.
For twenty months, from October 2014 to May 2016, the Large Underground Xenon (LUX) experiment was trying to detect dark matter. But at last week’s International Dark Matter Conference (a name I call immediate dibs on in case I start a girl band), Professor of Physics at Brown University Rick Gaitskell said: “What we have observed is consistent with background alone.”
The LUX experiment had failed. Dark matter remains as mysterious as Jess Mariano in season two of Gilmore Girls.
However, as the Sanford Underground Research Facility prepares for round two with an experiment 70 times more sensitive than LUX, now is a good time to talk about why dark matter is so darn hard to detect.
(Reminder: Isaac Newton’s law of universal gravitation states that gravitational pull is directly proportionate to the mass of the objects involved.)
We’ve suspected dark matter might be A Thing since 1933, when Swiss astronomer Fritz Zwicky studied the galaxies of the Coma Cluster.
The Coma Cluster is made up of around one thousand galaxies, and Zwicky found that they were moving way too fast “for the cluster to be bound together by the visible matter of its galaxies.” To account for the cluster’s gravitational pull, there had to be about six times more mass in the Coma Cluster than what he could see with his telescope.
Zwicky called that extra mass that would account for the Coma Cluster’s gravitational pull dunkle Materie, or dark matter. Without that unseen matter, the Coma Cluster could not conceivably have enough mass, and therefore gravity, to keep it in formation.
This is how the theory of dark matter was born, and it opened a can of worms that is still spilling forth to this day.
The truth is, in the words of the great Hank Green: “your chair, your friend, your planet, and your sister’s neighbour’s cow,”— that matter only makes up about five percent of what we know of the universe. The other 95 percent is dark matter and dark energy, and they’re both as uncharted as our deepest oceans here on Earth.
According to NASA, we know more about what dark matter isn’t, than what dark matter is. For a brief rundown, dark matter isn’t made up of: stars, planets, dark clouds of normal matter, matter made up of particles called baryons, antimatter, huge black holes, or neutrinos.
We know that dark matter doesn’t emit or absorb light, making it tricky to observe. But there is a foundation of science in favour of its existence.
Dark matter bends light with its gravitational pull, and this distortion of light has been captured by NASA’s Hubble Space Telescope. It shows how dark matter uses gravity to warp visible matter and space.
So what form does dark matter take? And how does it interact with gravity and normal matter? That’s what the LUX experiment was determined to find out, or at the very least narrow down.
There were two prime candidates for dark matter: MACHOs and WIMPs.
MACHOs, or massive astrophysical compact halo objects, have all but been ruled out of the race. They’re on the larger end of the scale— neutron stars, white dwarfs, objects drifting through space with no real sense of belonging to any particular solar system.
WIMPs, or weakly interacting massive particles, were the focus of the LUX experiment. They interact through weak force and gravity. This candidate tells a more persuasive tale—modelling shows there would be about five times more of these particles than quote/unquote normal matter. If WIMPs are the culprit, we’re practically swimming in the stuff.
Fritz Zwicky’s 1933 observation extends far beyond the Coma Cluster. It applies to other clusters, galaxies, and systems throughout the observable universe.
If the only gravity at play in our universe came from observable matter— the gravity generated from suns and planets— then the galaxies we’re observing now would have torn apart long before we ever had the chance to see them. If anything, dark matter is doing us a real solid by almost-probably existing.
When Fritz Zwicky answered his own questions— “How are these galaxies not spinning off into space? Why isn’t the observable mass equal to the gravitational pull?”— back in 1933, he opened the floodgates. 83 years later, the LUX experiment stands as the world’s most sensitive dark matter detector. Its failure to, you know, detect dark matter, is telling of how difficult the search for the invisible really is.