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Most of the universe's mass (roughly 85 percent) is some mysterious invisible stuff that we call “dark matter." It can’t be seen – hence the name – but we know it’s there because we can measure the mass of each galaxy, and all the normal matter (stars, planets, etc) aren’t nearly enough to make up that galaxy’s mass.
Imagine putting three 10-pound weights in a bag, then putting the bag on a scale, and it weighs not 30 but 100 pounds. There must be something else in the bag. The question is, what is it?
That’s what scientists are trying to find out. They’ve been looking in vain to find out what kinds of particles – if indeed they are particles – are flying around the universe unseen, making up the so-called dark matter. They expected to find something by now, but all searches have turned up empty. But they’re not giving up. In this post, we've outlined three ongoing searches for dark matter.
Since we don’t know what dark matter is, and since we have multiple theories as to what it might be, these searches aren’t necessarily looking for the same thing. But they all hope to solve the same mystery.
Theoretical physicists have come up with increasingly creative theories to explain our universe at fundamental levels. Some of these theories predict the existence of particles we haven’t yet observed. Some of these particles, if they exist, could make up dark matter. They’d fit the bill. But first, we need to do experiments to see which of these theories – if any – are correct.
Physicists hoped that by colliding particles together, faster than ever before, they could create some of the particles predicted in these theories. The Large Hadron Collider, the most powerful particle accelerator ever created, was built in the hope that new particles might be discovered. If so, that might solve the dark matter mystery by showing that a particle exists that fits the bill.
However, despite an upgrade to the LHC completed in 2015, it still failed to find any new particles that could explain dark matter. Most recently, in August, an LHC search failed to find “dark quarks." But there’s hope on the horizon: in June, it was announced the LHC is getting another upgrade – this one could give it the precision needed to detect particles slipping through the cracks of their current experiments.
The fact that we can’t see dark matter, whatever it is, means that light probably doesn’t bounce off of it to reach our telescopes. And that implies that dark matter probably doesn’t interact much, if at all, with the electromagnetism that makes up light, if at all.
That means dark matter can probably pass through the Earth, and through you, as if nothing was there. That’s because it’s electromagnetism that normally makes things bounce off each other, so a particle that doesn’t interact with electromagnetism won’t be stopped when it bumps into an electromagnetic particle (like the atoms that make up the Earth). That’s pretty cool, but it also means that dark matter is hard to find. It’s hard to build a detector for a particle if that particle can go right through your detector like a ghost.
But physicists are hopeful that dark matter does interact with normal particles at least a little bit – maybe, very rarely, one of them could bump into particles in our atmosphere, as it flies in from space. If so, the collision could create other particles, which we could detect. The problem is filtering out all the noise from ordinary particles which also hit our atmosphere. Luckily this is not an impossible task and a number of cool projects are doing just that – like XENO1T, an underground, 3500 kg tank of xenon – sadly, with no detections so far.
One promising particle that could be dark matter is called the axion, and if axions exist in our galaxy and are passing through the Earth, it might be possible to catch them. That’s because in the presence of a strong magnetic field, axions would be expected to turn into microwaves, and microwaves are something we know how to detect.
The Axion Dark Matter eXperiment is looking for axions this way, hoping they’ll turn out to be the culprit in the dark matter mystery. Sadly, they’ve so far turned up no positive results. Their recent results came closer than before to finding axions, if they’re there, or ruling them out. The researchers believe the next round of results should provide “nearly definitive tests” of whether the axion exists. So one way or another, we should hopefully find out soon!
This has been a sampling of the multitude of ways physicists are looking for dark matter. Dark matter is one of the most baffling puzzles facing physicists and it’s going to take a lot of creativity, and a lot of effort, to find the true identity of the mysterious matter. One of the present searches might find dark matter or it might be something no one’s thought of yet. A small minority of physicists think it might not be matter at all – but that a modification to our theory of gravity might explain it. Only time, and a lot of determination, will tell
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