Did Most of the Universe Go Missing?
Seventy-one years ago, most of the universe went missing. According to many astronomers, it's still missing.
The problem started with the famous Swiss physicist Fritz Zwicky. It was he who coined the word supernova. He was such a heavy hitter, everyone paid attention when his gigantic brain went Boing.
It did exactly that in 1933, when he studied speeds in a group of galaxies. What he perceived was astonishing. Each member moved so quickly, it should have no problem escaping the gravitational glue of the entire assembly. Zwicky realized that this galaxy cluster—and all others, it soon turned out—shouldn't even exist. Yet there they were.
Extra gravity must lurk within and among the galaxies. The conclusion was bewildering: the universe is apparently dominated by some invisible substance boasting an enormous “pull.” Zwicky called it dark matter and the name stuck.
As telescopes got bigger and we observed more of the universe, the situation stubbornly endured. There's far more gravity in the universe than can be accounted for. What's creating it? What is this strange, unseen, powerful stuff?
Most astronomers assume dark matter consists of undiscovered particles that only anemically interact with planets, stars, our bodies, ourselves. They call this WIMPS, for Weakly Interacting Massive Particles.
It's not farfetched. The universe's most common particle is the neutrino. These too are invisible. These, too, are numerous, and barely influence normal matter. A trillion neutrinos pass through each of your fingernails every second. So it's not much of a stretch to imagine a kind of souped-up neutrino mostly dwelling in haloes around each galaxy. Meaning, galaxies are like ships in bottles, enclosed by dark, massive, unseen spheres.
Every few years, researchers think they've spied traces of WIMPS, but it's never panned out. Then, almost forty years ago, an Israeli physicist proposed an entirely different solution to the crazy galaxy motion problem. Instead of seeking missing mass that tugs at everything, he showed that we'd see the same thing if gravity itself behaves differently at weak levels.
If there's a lower limit on how wimpy gravity can become, then the motion of the universe makes sense without there needing to be any dark matter at all.
So which is it? Weird gravity, or some unseen substance? The food fight between the two sides is likely to rage for years. Until then, we can only gaze into the night sky and wonder whether most of the cosmos is really missing.