The night sky is packed with galaxies.

Just from Earth, astronomers can see hundreds of billions of them, and estimates suggest there could be trillions in total.

That's awesome, not only because big numbers are really fun to say, but because when you have a lot of something, even really rare events end up happening.

We see that idea in play all the time with stars and planets, but entire galaxies can also be super strange.

Here are three of the most extreme galaxies scientists have discovered so far.

You won't believe your eyes on this first one, because your eyes literally won't see anything at all.

It's Dragonfly 44, the invisible galaxy, and it was picked up by a survey telescope back in 2015.

Okay, so it's not quite invisible, but it is 100 times dimmer than the Milky Way would look at the same distance, even though it's about the same mass.

Astronomers call objects that are large in size, but low in brightness like this ultra-diffuse galaxies.

Despite its size, this one is so dim because it's made of vast quantities of dark matter.

Dark matter is one of the most important mysteries in modern-day astronomy.

Measurements indicate that it makes up more than 80% of all the matter in the universe, but scientists have no clue what it is.

It's called dark because it doesn't emit or absorb light.

We only know it exists because of the enormous gravitational influence it has.

Astronomers know that almost every galaxy, ours included, has more dark matter in it than regular, visible stuff.

What makes Dragonfly 44 so special is that it's nearly all dark matter.

Observations indicate it might be 99.99% dark matter, way more than any other known galaxy.

We don't really know how so much of this stuff ended up in one place, but answering that question could be a big deal.

Scientists are still puzzling over how galaxies form in the first place, but the presence of dark matter seems like a key element.

So having such an extreme case to study is super useful.

Another piece of the galaxy formation puzzle is why some galaxies look like they do.

Astronomers have long lumped most galaxies into one of two main categories, spirals and ellipticals, and they have a pretty good idea how each kind forms.

But there's actually a third, rarer category called lenticular galaxies that seems sort of halfway between a spiral and an elliptical.

Galaxy UGC 12591 takes that one step further. It looks kind of halfway between a spiral and a lenticular.

It has the fluffy, lens-like central bulge that gives lenticular galaxies their name, but also the dark lanes of dust and flat disk characteristic of a spiral.

If you sort of squint, you might even be able to imagine that it has spiral arms wrapped up really tight, like the waves in Saturn's rings.

This galaxy also has one other neat feature: It spins stupid fast, almost twice as fast as the Milky Way.

With a rotation speed of 1.7 million kilometers per hour, it's the fastest known disk-shaped galaxy.

Once again, astronomers don't really know, but since not much is known about lenticulars in general, even a little new information could be helpful.

Now, I promised you three extreme galaxies, but this last one is actually a pair of them with a single name: ADFS-27.

When it was spotted, scientists realized this thing was both incredibly bright and incredibly far away, almost 12.7 billion light-years.

It's so far that its light has taken most of the universe's history to get to us, so we're seeing this object as it was in the distant past.

And in 2017, we finally got a better view of it thanks to the ALMA radio telescope.

It turned out to be a massive galaxy collision taking place just a billion years after the Big Bang.

And it might be the largest galaxy merger ever observed.

We're talking huge, like it makes Godzilla seem puny.

Each galaxy is more than a dozen times larger than the Milky Way, and we see them just 30,000 light-years apart, or about a hundred times closer than the nearby Andromeda galaxy is to us.

And if that wasn't enough, both belong to a rare group of objects called starburst galaxies.

I wish I could say they're made of actual Starburst candies because that would be delicious, but instead, they just make a lot of stars. Which is also cool.

Together, these galaxies contain more than 50 times the star-forming material as our Milky Way, and they're producing stars 1,000 times faster.

And that intense star formation is being driven by their collision, so it will probably keep getting brighter over millions or billions of years.

The end result is likely an elliptical galaxy so massive that it will form the gravitational anchor of its own galaxy cluster.

By now, billions of years later, other galaxies are probably orbiting what's left of this giant collision.

If only light traveled faster, we might be able to see it for ourselves.

And if there's a theme to all this, it's that we'll probably never know exactly what happened to make each of these galaxies look the way they do today.

But you learn new things not just by looking at the seemingly normal cases, but by studying those that seem like they don't make any sense.

And, hey, if you smash together a few galaxies along the way, it's just good, clean fun!

Speaking of good fun and learning, right now Brilliant is offering the first 200 SciShow Space viewers to check out Brilliant.org/SciShowSpace a 20% discount on an annual premium subscription.

That subscription gives you access to tons of interactive courses, as well as the huge Brilliant community.

If you liked this episode, I recommend you check out the Astronomy Toolbox quiz on Matter.

It stretches your brain, and the exploratory spirit of how the quiz is written is really in line with how I feel when I'm thinking about space.

And if you consider yourself an advanced astronomer, check out the course on Spacetime and Paradoxes.

That one's next on my list and I'm excited to take a crack at it.

Thanks for watching and check out Brilliant.org/SciShowSpace to take advantage of that 20% discount offer.