This Week's Amazing Sky: Dark Matter!

June 6, 2016
Dark Matter

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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 clusterand all others, it soon turned outshouldn'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.

Credit: HubbleSite

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.

Credit: HubbleSite


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.

Credit: NASA

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.

About This Blog

Welcome to “This Week’s Amazing Sky,” the Almanac’s hub for everything stargazing and astronomy. Bob Berman, longtime and famous astronomer for The Old Farmer’s Almanac, will help bring alive the wonders of our universe. From the beautiful stars and planets to magical auroras and eclipses, he covers everything under the Sun (and Moon)! Bob, the world’s mostly widely read astronomer, also has a new weekly podcast, Astounding Universe

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There are so many mysteries

There are so many mysteries left in life--in the atom, in the universe, in our own minds. It seems to go hand in hand with beauty.

Remember the definition of

Remember the definition of "invisible" -- it means only that light (wavelengths our eyes can detect) does not interact with it. The term "darK" is perhaps more descriptive than "invisible", as it connotes something we have no knowledge of. Yet, that is. Our mathematical models may lead us to a way to understand what's going on. I think the answer will surprise us, when we learn it.

Interesting article but is it

Interesting article but is it 81 years or 1943 that this occurred.

Hi -- you mean 1933, right?

Bob Berman's picture

Hi -- you mean 1933, right? Sorry I didn't make the chronology clear. I'll do it now. In the 1920s, we first discovered the speeds of external galaxies. In 1933, Zwicky showed that something about them didn't make sense. Since then, we've made periodic confirmations of this "dark matter."  

Actually, I meant did you

Actually, I meant did you mean 71 years which would be 1943 or 81 years which would be 1933. However, thank you for your explanation.

I read that we only know 5%

I read that we only know 5% of our universe is matter that we can see. That's an awful lot of invisible stuff.

Yes, you're right. As bad as

Bob Berman's picture

Yes, you're right. As bad as "dark matter" is, which is an unseen pulling (gravity-like) force, there's also "dark energy", a mysterious unseen repelling force, which appears to constitute most of the cosmic inventory. Kind of humbling, to be ignorant of most of the universe.


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