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The hunt for dark matter is still on, and the candidates for it could be primordial black holes as massive as Earth, or axions, as tiny as the smallest subatomic particles in existence!

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Dark matter is some cosmically strange stuff.  We can't see it, but it appears to outnumber all the stuff we can see, all the stars and planets and random dust floating around by five to one.  We only know or think we know it's there because of the gravitational effect it has, and astronomers have spent decades trying to figure out what kind of strange stuff this dark matter could be made of.  One candidate is a bunch of regular matter our equipment just isn't sensitive enough to detect, but the hunt also includes objects that are a little more exotic.  

A handful of papers published within the past couple weeks dive into that hunt and show that it's far from over.  One of these candidates for dark matter is a group of black holes that formed long before the first stars ever shone.  They're called primordial black holes, or PBHs, but there's not much actual evidence for them yet.  They're still hypothetical.  The black holes we typically think of form after a really massive star goes supernova, but that doesn't have to be the only way.  

It's been theorized that not long after the big bang, small random pockets of space just happened to have enough matter stuffed into them that they collapsed into black holes.  Because they didn't start out from stars, they could come in an extreme range of masses, from fractions of a gram up to many, many times that of a star, and that also means that unlike black holes that started as stars, they're not likely to be surrounded by large swaths of gas and dust, which would make them really tough to spot.

Some scientists, including Stephen Hawking, have suggested that these so-far undetected black holes could make up at least some of the universe's dark matter.  Previous research had already found that they can't account for all dark matter, though, so new research out of Japan published this week in the journal Nature Astronomy set out to determine how abundant they might be.  

To find these black holes, the team searched galactic halos.  That's the region beyond the disc, which doesn't have nearly as many stars, gas, and dust, and is where astronomers suspect primordial black holes could invisibly hang out.  When a star passes between us and dark matter, it's like it's bent and magnified.  

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So to hunt for PBHs, the team monitored the change in brightness of over 10 million stars in the outer fringes of the Andromeda galaxy and the Milky Way.  After controlling for everything they could think of that would cause a dip in brightness, including one unwelcome asteroid, there was only one possible event that might be attributed to a PBH.  Annoyingly, even that one could just be a variable star that dips in brightness naturally, but they couldn't be sure, but running with the assumption that they were able to observe a single potential PBH, the team was able to place an upper limit on just how common these things could possibly be, at least in the range of masses they looked for.

If PBHs made up all dark matter, they would have expected to see about 1,000 brightness dips over the course of their experiment.  Seeing only one, they say, means that those PBHs could only account for a small fraction of dark matter, maybe 0.1%.  Rather than black holes, other teams are working to find an incredibly tiny hypothetical particle called the axion.

Axions, if they exist, are fundamental particles.  That means that, like electrons, they're not made up of any constituent particles, and like neutrinos, they have almost no mass and don't interact with ordinary matter very often, and even weirder, they can actually bend the rules we expect electricity and magnetism to play by, at least by a teeny tiny amount.  Because of that, a team based out of MIT designed a laboratory experiment to try and detect these changes.

They call it ABRACADABRA, which might have been appropriate if they could have pulled dark matter out of it or if it had been shaped like a top hat, but instead, it's a donut-shaped magnet roughly the size of a basketball inside an insulated refrigerator.  Under the standard laws of electromagnetism, there's no magnetic field in the donut hole, but if an axion is there, there should be.  

It would be super duper weak, though, roughly a trillion times weaker than the Earth's magnetic field, so the team had to design their experiment to block out everything that could drown out that signal, from the local radio stations to LEDs on nearby electronics, and they had to keep the magnet as cold as possible, just above absolute zero.  

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After running the experiment for a month, they were unable to detect evidence of axions, at least none within their target mass range, around a quintillionth the size of a proton.  So that either means they don't exist or their influence on electromagnetism isn't as strong as predicted, but this experiment was only a prototype, so a larger and better shielded one will be able to hunt for smaller axions.  They just haven't built it yet, but we don't necessarily have to build contraptions to find axions.  We can try to find them in space.

On an astronomical scale, a large number of axions together could basically act as its own field.  Think of it like a cosmic ballpit made of incomprehensibly tiny floating balls.  Stuff passing through would theoretically interact with these axions, at least a little, especially light, otherwise known as electromagnetic waves, and passing through the field would affect its polarization or the angle of the light waves, and that is something we could potentially observe. 

So in another recent paper published in the Journal of Cosmology and Astroparticle Physics, one team based in Russia went hunting for that polarization.  They looked at objects called active galactic nuclei, which are dense galactic centers that spew out tons of polarized electromagnetic radiation.  Specifically, they were looking to see if the polarization of the light coming from their targets changed over time and all in the same way, so if they observed the predicted changes, that would mean that the light from all the different active galactic nuceli was being affected by an external field made up of a very large number of very light axions, but they didn't see that.  They, too, were unable to conclude whether or not axions exist, at least for their study's mass range, which, incidentally was way smaller than the one probed by ABRACADABRA.  

All of this work helps to place limits on what masses dark matter particles can have or what they can even be.  It might be primordial black holes as massive as the Earth and it might be as teeny tiny as the smallest sub-atomic particles in existence, or something else entirely.  Our technology just hasn't caught up with our curiosity yet.  It just means that the hunt for dark matter is still on, and I can promise you that when scientists figure it out, you'll hear about it here.

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