This episode of SciShow Space  is sponsored by MagellanTV, a documentary streaming  service, founded by filmmakers.

SciShow Space viewers will get a  one-month free trial on MagellanTV by clicking on the link in the description. [♪ INTRO] Even if you feel like you’re standing  still on the surface of the Earth, you never really are. Our planet is spinning, and it’s orbiting the Sun.

But the entire solar system is  also hurtling around the galaxy, orbiting the supermassive  black hole at its center. And while most of us are thankful that  we can’t feel these dizzying motions, some scientists are hoping  to use our galactic flight to help detect some of the most  elusive particles of all: dark matter. Interstellar space is far from empty.

In fact, it has some particles that are  potent, but invisible, like dark matter. And although we haven’t seen it, Earth might be experiencing  some of it as headwinds. You see, as a solar system, we’re  whirling around the center of the galaxy at a whopping 220 kilometers per second.

Adding all these motions together,  the Earth is moving fastest through the galaxy during June,  ​when the planet’s orbital motion aligns with the sun’s motion  through the Milky Way. Whereas in December, the  Earth is moving ‘backward’ compared to the Milky Way’s spin, kind  of like swimming against the current, so our overall speed is at its lowest. Luckily for us, most other things in the Milky Way are also wheeling around the  galaxy at a similar rate, so we’re not in much danger of  barrelling into another star or planet.

But although it might seem like a black void, we might be plowing through a lot of dark matter. Now, dark matter is thought to outweigh  normal matter by about six to one. But we’ve never managed to pinpoint  the particles responsible for it.

Something powerful but invisible  must be exerting a force on the stars orbiting in galaxies, or  else those galaxies would fly apart. So it’s thought that there’s a shell or  halo of dark matter that sits like a cloud over the galaxy, providing the gravity  needed to keep those stars in check. If that’s correct, then as we orbit the Milky Way, we are plowing through this dark matter  slightly faster in June than in December.

And the Earth is experiencing a  dark matter headwind as it travels. Compared to the motion of the stars and  planets, dark matter is pretty much static, so the strength of the headwind we experience depends on the speed of Earth at any given time. Just like when you stick your hand out  of a car when you’re driving along.

You feel a wind from all the air  molecules your hand is hitting. The faster the car, the faster the wind feels. So as the relative speed of our  planet changes through the year, so too does the strength of  the dark matter headwind.

Meaning that we should experience  more of that headwind during June, when we go the fastest, and less in  December when we’re at our slowest. And now, an experiment deep in  the Italian Apennine mountains claims to have detected this  variable headwind signal, with exciting implications for  the detection of dark matter, too. The experiment, named DAMA, has been running at the Gran Sasso National Laboratory  in Italy for about 20 years.

They use crystals of sodium iodide  doped, or impregnated, with thallium. These crystals are often used to detect  radioactivity in lots of different scientific fields, like medicine,  geophysics, and nuclear physics.   So when any subatomic particle or  strong electromagnetic radiation strikes these crystals, they absorb that  radiation and release the excess energy as a photon, which we see as a flash of light. The tricky part here is that Earth gets  lots of radiation from things like the Sun and space in general, so  researchers had to prevent all the sources of radioactivity  from getting into the crystals So you can imagine they’re heavily shielded.

The crystals are engulfed in  a pure nitrogen atmosphere, surrounded by thick layers of  copper, paraffin, and concrete, buried under nearly one and a  half kilometers of overlying rock. And even with all these protective layers,  the crystal detectors still emit light. A signal that rises and falls over  the year, with more flashes in June, and fewer seen in December.

Kind of on  schedule with the projected headwinds. So if the shields can block out  all known particle bombardments, then the scientists conclude that the  variable signal they are detecting must be coming from something we  have not yet seen and accounted for. And the best candidate for that  is the dark matter headwind.

After 20 years of measuring, the  team is certain that their signals are real and not random background noise. But confirming the detection of dark  matter isn’t so straightforward. There’s a bit of controversy  surrounding this result.

Many scientists are skeptical that  the annual signal is really coming from beyond the solar system. Some suggest it could be  something unaccounted for, that changes with the seasons and  not with the headwinds per se. And other similar experiments have failed  to find the annual variation at all.

But a new project on the other side of  the world hopes to combine its results with DAMA, to test the galactic  headwind hypothesis once and for all. It’s called SABRE, and it’s located at the bottom of an active gold mine in southeastern Australia. The experiment will use the same setup as DAMA, and hopes to find the same annual  variation that the Italians have detected.

But the timing of that rise  and fall will be critical, given that this other experiment  is in the southern hemisphere. If the detectors peak during the  Australian summer, in December, then it is likely that the annual  variation is down to the changing seasons. But if the detections peak in June,  as it does in the Italian experiment, then it is more likely to be caused  by something beyond the Earth, like the dark matter headwinds.

And if they don’t detect  any annual variation at all… well, it’ll be back to the drawing board! Solving the mystery of dark  matter is never going to be easy, but these cutting-edge experiments  are showing the power of thinking outside the box, or at least  outside the solar system! Thanks for watching this episode of  SciShow Space, if you want to learn more about the mysteries of dark matter,  check out today’s sponsor MagellanTV.

MagellanTV is a documentary streaming  service, founded by filmmakers. So they have a bunch of high-definition  content that goes in-depth on different science topics like  space, technology, nature, and more! For example, they have a  whole hour-long documentary on the mysteries of dark matter, where  you’ll learn more about the scientific quest surrounding this strange unknown substance.

So if you like space as much as we  do, we know you’ll like this one. The best thing is that they  can be watched anytime, anywhere, on your television,  laptop, or mobile device. So click the link in the description  for your first full month free. [♪ OUTRO]