We can only see about 4 percent of our universe. The rest is mysterious, invisible dark energy
We can only see about 4 percent of our universe. The rest is mysterious, invisible dark energy

and dark matter, which scientists still don’t know very much about.
and dark matter, which scientists still don’t know very much about.

Obviously, they’d like to learn more about the other 96% of the universe. But how do
Obviously, they’d like to learn more about the other 96% of the universe. But how do

you study something you can’t see?
you study something you can’t see?

Well, one option is to study the dark parts of our universe indirectly, by looking for
Well, one option is to study the dark parts of our universe indirectly, by looking for

ways they might be influencing the visible universe.
ways they might be influencing the visible universe.

And earlier this year, an international group of astronomers confirmed that some of those
And earlier this year, an international group of astronomers confirmed that some of those

clues pointed to a small, undiscovered galaxy, mostly made of dark matter, orbiting our own galaxy.
clues pointed to a small, undiscovered galaxy, mostly made of dark matter, orbiting our own galaxy.

It’s being called Galaxy X.
It’s being called Galaxy X.

Back in the 1970s, astronomers were measuring the rotational speeds of galaxies, and they
Back in the 1970s, astronomers were measuring the rotational speeds of galaxies, and they

noticed something surprising: the objects as the edges of galaxies had velocities that
noticed something surprising: the objects as the edges of galaxies had velocities that

didn’t fit with the astronomers’ calculations.
didn’t fit with the astronomers’ calculations.

The researchers were predicting the movement of objects in the galaxy based on the amount
The researchers were predicting the movement of objects in the galaxy based on the amount

of visible matter, and its gravitational influence. But these things were moving way faster than
of visible matter, and its gravitational influence. But these things were moving way faster than

they should have been.
they should have been.

They realized that for the velocities of these objects to make sense, there had to be gravity
They realized that for the velocities of these objects to make sense, there had to be gravity

from invisible, dark matter affecting them too.
from invisible, dark matter affecting them too.

More recently, astronomers have found small, dim galaxies, like the Draco Dwarf Galaxy
More recently, astronomers have found small, dim galaxies, like the Draco Dwarf Galaxy

and Triangulum II, that appear to have much more gravitational force than they should,
and Triangulum II, that appear to have much more gravitational force than they should,

suggesting that they’re packed with dark matter.
suggesting that they’re packed with dark matter.

Physicists have even guessed that there might be galaxies mostly, or entirely, made up of
Physicists have even guessed that there might be galaxies mostly, or entirely, made up of

dark matter: dark matter galaxies.
dark matter: dark matter galaxies.

Back in 2009, two researchers -- Sukanya Chakrabarti and Leo Blitz -- analyzed variations in the
Back in 2009, two researchers -- Sukanya Chakrabarti and Leo Blitz -- analyzed variations in the

distribution of gas at the edges of the Milky Way, and they found that these so-called “ripples”
distribution of gas at the edges of the Milky Way, and they found that these so-called “ripples”

could be caused by a clump of dark matter orbiting our galaxy.
could be caused by a clump of dark matter orbiting our galaxy.

In other words, they’d found evidence of a new dwarf galaxy, about a hundredth the
In other words, they’d found evidence of a new dwarf galaxy, about a hundredth the

size of our own and made almost entirely of dark matter.
size of our own and made almost entirely of dark matter.

If it’s really out there, Galaxy X would have skimmed the edge of our galaxy a few
If it’s really out there, Galaxy X would have skimmed the edge of our galaxy a few

hundred million years ago, and would now be about 300,000 light-years from the Milky Way’s
hundred million years ago, and would now be about 300,000 light-years from the Milky Way’s

center -- about 250,000 light-years farther than the galactic rim.
center -- about 250,000 light-years farther than the galactic rim.

But that’s the thing -- they didn’t know if it was out there. The math made sense,
But that’s the thing -- they didn’t know if it was out there. The math made sense,

but they needed more evidence.
but they needed more evidence.

Since they had a basic idea of where Galaxy X should be, they and a few other researchers
Since they had a basic idea of where Galaxy X should be, they and a few other researchers

looked for clues in that area, in the form of the small amount of visible matter that
looked for clues in that area, in the form of the small amount of visible matter that

might live in the mostly-dark galaxy.
might live in the mostly-dark galaxy.

And in 2015, they published a paper saying that they’d found 4 of a kind of star called
And in 2015, they published a paper saying that they’d found 4 of a kind of star called

a Cepheid variable in that area of the galaxy.
a Cepheid variable in that area of the galaxy.

Cepheid variables are pulsating stars, meaning that they change in apparent brightness -- the
Cepheid variables are pulsating stars, meaning that they change in apparent brightness -- the

amount of their light that reaches Earth -- in a predictable way.
amount of their light that reaches Earth -- in a predictable way.

By measuring that change in brightness, astronomers can calculate how much light the star is actually
By measuring that change in brightness, astronomers can calculate how much light the star is actually

putting out -- its luminosity.
putting out -- its luminosity.

And from there, they can figure out the distance from Earth to the star.
And from there, they can figure out the distance from Earth to the star.

All four of the stars were about 300,000 light-years from the center of the galaxy, and at roughly
All four of the stars were about 300,000 light-years from the center of the galaxy, and at roughly

the same angle as the rest of the galactic disk -- just about where Galaxy X was predicted to be.
the same angle as the rest of the galactic disk -- just about where Galaxy X was predicted to be.

Of course, these stars could still be part of our own galaxy.
Of course, these stars could still be part of our own galaxy.

But that would be weird, because no other Cepheid variables have been found that far
But that would be weird, because no other Cepheid variables have been found that far

from the center of the galaxy, but still close to the angle of the galactic disk.
from the center of the galaxy, but still close to the angle of the galactic disk.

And the researchers found four clustered together!
And the researchers found four clustered together!

So it’s possible that these stars are some of the bits of visible matter in Galaxy X.
So it’s possible that these stars are some of the bits of visible matter in Galaxy X.

The team’s next step was to figure out if the stars were moving with the same speed
The team’s next step was to figure out if the stars were moving with the same speed

they predicted for Galaxy X -- around 200 kilometers per second.
they predicted for Galaxy X -- around 200 kilometers per second.

And in January of this year, they announced that they’d analyzed 3 of the Cepheid variables
And in January of this year, they announced that they’d analyzed 3 of the Cepheid variables

-- the ones they had data for -- and they are moving at about the same speed as Galaxy X should be.
-- the ones they had data for -- and they are moving at about the same speed as Galaxy X should be.

We still don’t know for sure if these stars mark the location of Galaxy X. But if researchers
We still don’t know for sure if these stars mark the location of Galaxy X. But if researchers

find enough evidence to conclude that they do, then we’ll have found a dark matter
find enough evidence to conclude that they do, then we’ll have found a dark matter

galaxy by studying the ripples in the gas and dust at the edge of the Milky Way.
galaxy by studying the ripples in the gas and dust at the edge of the Milky Way.

And maybe, if astronomers spend more time analyzing these ripples, they’ll find more
And maybe, if astronomers spend more time analyzing these ripples, they’ll find more

dark matter galaxies, and help unveil a tiny bit of the darkness that dominates our universe.
dark matter galaxies, and help unveil a tiny bit of the darkness that dominates our universe.

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