In case you haven't heard, there is a very, very big problem with the universe: About 80% of all of the stuff inside it is missing.
Astronomers call this material "dark matter." They know it's out there because its huge mass tugs on and shapes galaxies, but no one has ever detected the material itself. Aside from exerting a gravitational pull, dark matter doesn't seem to interact with stars, planets, dust, atoms, subatomic particles, or any other "normal" matter as we know it. It's essentially invisible.
Yet this deep cosmic mystery, which has perplexed scientists for nearly 85 years, may soon be coming to a close — possibly within the next year.
A group of researchers led by NASA cosmologist Alexander Kashlinsky thinks the recent and groundbreaking discovery of gravitational waves could help rule out the idea that dark matter is made of exotic, hard-to-detect particles.
Their suspicion: Massive black holes, like the two whose collision caused the gravitational waves, are far more common than suspected and might have formed in the first fraction of a second of the Big Bang. That could mean the dark matter that makes up most of our universe is not exotic particles at all. It might simply be black holes.
If Kashlinksy and his colleagues are correct, the reality may be that four-fifths of matter was gobbled up by black holes at the dawn of the universe, millions of years before the first star was born.
And there's another uncomfortable facet of this hypothetical reality: A countless number of ancient, greedy cosmic drains are lurking in the blackness of space around us, pulling on "normal" matter like faceless puppeteers.
Origins of a dark mystery
Fritz Zwicky, an astronomer at at Caltech, first proposed the idea of dark matter in 1933.
Zwicky was trying to nail down the motion of a now-famous group of galaxies called the Coma Cluster, which is made of a trillion-or-so stars and exists some 2.2 billion light-years from Earth. To do that he compared two measurements of the cluster: its brightness and how fast its galaxies were moving.
After Zwicky photographed the cluster with a special telescope that Caltech built for him, he measured the average speed of all those galaxies, which — per Einstein's theory of relativity — gave him the weight of the cluster. Then he compared that data to the cluster's brightness.
Zwicky was dumbfounded. The cluster was much too dark to explain the motions of all the galaxies; his calculations showed the groups of stars were much too heavy.
He reasoned that some unseen mass, which he called "dark matter," was lurking in the cluster and bulking it up, thus explaining the discrepancy.
Researchers ever since have found similar pockets of dark matter all over the universe, lurking at the edges of galaxies in massive "halos" of material and toying with the movement of their stars.
They've also proposed two powerful ideas to explain what dark matter is — but only one of them might be correct.
MACHOS vs. WIMPs
Black holes quickly became a prime suspect for being dark matter.
After all, they don't emit any light and are unbelievably dense and compact — pretty much the perfect objects to secretly exert a powerful gravitational force on visible matter.
Calculations like Zwicky's suggested they might lurk at the periphery of galaxies in rings, so astronomers came to call them "massive compact halo objects," or MACHOs.
Scientists figured MACHOs probably formed within a fraction of a second of the Big Bang. This was right as the event's hot, energetic soup of plasma cooled into the first subatomic bits of matter (then eventually into atoms, gases, stars, and so on).
These primordial black holes would have lurked in the background as the first stars, galaxies, and planets started to form millions of years later, subtly influencing their destinies while vastly outweighing them; for every bright, shining star, four stars' worth of mass was forever locked away in a black hole.
They also would have left an imprint on the deepest, oldest images of the universe, called cosmic background radiation. The radiation is a faint afterglow of energy left some 400,000 years after Big Bang, when subatomic particles finally condensed into atoms and cooled off a bit (thus emitting all the light we can detect today).
But the idea of MACHOs being dark matter soon fell out of favor.
"There was a suggestion made a long time ago about dark matter being lots of black holes formed early on in the universe," Vicky Kalogera, an astrophysicist at Northwestern University, told Tech Insider. "[I]f this were true, we would've seen certain signatures in the cosmic microwave background. We don't see those kind of signatures."
(The cosmic microwave background, or CMB, is the best-known radiation fingerprint of the early universe.)
Researchers have also looked for gravitational lenses, or the warping of space (and light) caused by massive objects like black holes. We've seen plenty of lenses with larger objects, like the galaxies shown above.
Evidence for "microlensing" that primordial black holes might cause by, say, passing in front of a bright star, has yet to be found.
So in recent years, a more popular idea emerged that dark matter could be made of weakly interactive massive particles, or WIMPs: pieces of matter that are incredibly heavy yet frustratingly difficult to detect because they pass through almost all baryonic (normal) matter like a ghost.
But the situation has become increasingly shaky with WIMPs.
Several ambitious experiments underway right now use supercooled detectors deep underground to hunt for signs of these dark matter particles. But instruments like the LUX experiment, situated far below the the Black Hills of South Dakota, have recently made the news for failing to detect any evidence, even after years of looking.
"Such searches have been going on for quite some time, but nothing has been reliably detected, slowly shrinking the box of parameters where [dark matter] particles can hide," Kashlinksy wrote in an email.
Which is why the discovery of gravitational waves got Kashlinksy and his colleagues so excited.
Colliding black holes and microlenses
In February 2016, researchers detected ripples in the fabric of space called gravitational waves, a discovery that, until then, had eluded Earth's brightest minds and most sensitive machines for almost 100 years.
The cause of the waves? An epic collision of two black holes, one the weight of about 29 suns and the other 36 suns. Some three suns' worth of matter instantly turned into gravitational wave energy during the event, warping spacetime in a way that scientists could detect on Earth more than a billion years afterward.
At the heart of that discovery was the Laser Interferometer Gravitational-Wave Observatory (LIGO), a 15-nation, 900-scientist, $1 billion experiment, that has searched for signs of the phenomenon since 2002.
The size of those black holes — again, about 30 suns' worth of mass — is supposed to be rare. And that has reinvigorated the idea dark matter is actually made up of MACHOs: a theory that, perhaps, was discounted too quickly.
Kashlinksy says objects of that size not only fit the bill for MACHO size estimates, but also objects of that size haven't definitively been ruled out in the search for microlenses. What's more, he added, cosmic background radiation data may not be detailed enough to see the disturbances caused by ancient black holes.
"This is like what I show my kids when I make pizza: You spread the dough and suddenly see holes appearing in the dough. That may have happened at very early times," he said, before the first matter began to form.
Kashlinksy may have found some hint of those MACHO "dough holes" in the universe's ancient radiation by comparing cosmic infrared background (CIB) maps to cosmic X-ray background (CXB). In short, there is some unexplained patchiness — a possible signature left by primordial black holes.
A second discovery of gravitational waves, announced in June 2016, didn't jibe as well with Kashlinksy's theory, since the two colliding black holes — one 8 solar masses, the other 15 — sit near the lower, 10- or 20-solar-mass minimum of what primordial black holes should weigh.
But this hasn't dissuaded Kashlinksy.
"There are limits of what primordial black holes could be, on the order of up to 1,000 solar masses," Kashlinksy said. "LIGO black holes fall into this range. Maybe this is what the dark matter is."
He added: "We should know the answer within a couple of years. LIGO is starting a new run of observations in September. If they keep discovering these beasts at this rate, every one to two weeks, we should have good enough statistics to rule in the possibility."
Proving the case
Meanwhile, Kashlinksy and his collaborators are not sitting idly by.
One idea they've recently proposed may be a better way to see microlenses caused by black holes formed at the dawn of the universe.
"If you look at our own galaxy, there should be some 10 to 100 billion primordial black holes. Occasionally some of these should pass in front of a star" and you should see the effect, he said.
The team is also working on an instrument for a cosmic background telescope called Euclid that's currently scheduled to launch in 2020.
The device, called LIBRAE, should be vastly more sensitive than COBE — the NASA space telescope that, so far, has recorded the best maps of cosmic background radiation — and create maps fine enough to show those "pizza dough holes" that any primordial black holes left during the early universe.
Kashlinsky feels strongly that he could be correct about black holes solving the riddle of dark matter, but noted he's "not a betting man."
"On the dark matter particle side of the spectrum, the range of possibilities is narrowing down quickly," he said. "If nothing is found there, and nothing is found in the black hole theater, then we may be in a crisis of science."
Sarah Kramer contributed to this post.
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