WEBVTT FILE

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We all know what a galaxy looks
like, right? It’s a huge

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collection of stars and other
matter that’s shaped like a

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spiral or an ellipse, and if
you’re an astronomy fan you

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probably know that most of the
mass is from invisible,

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mysterious material called dark
matter. Well, NASA’s Hubble

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Space Telescope just took an
image of a galaxy that is none

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of those things. For the first
time, astronomers have strong

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evidence for a galaxy not having
a significant amount of dark

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matter. Most astronomers
currently believe that dark

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matter plays a fundamental role
in our universe and the

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formation of galaxies. This is
because galaxies seem to have a

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lot more mass than what we can
account for based on just the

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stars we see. How much mass is
in a system is determined by

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measuring the speed at which
galaxies rotate or individual

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stars in a galaxy move. Without
the gravity from that mass, a

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galaxy would fly apart if
everything in it is moving as

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quickly as we observe it moving.
Decades of research have led

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astronomers to the extraordinary
conclusion that 85% of the mass

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in our universe is invisible in
all wavelengths of radiation,

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and that it’s composed of matter
that does not contain protons or

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neutrons or any type of particle
we’ve detected before, AND that

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this invisible material is all
around us, passing through us

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without interacting with regular
matter except by gravity. For

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some, that’s a tough pill to
swallow, and a minority of

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astronomers wonder if maybe we
just don’t completely understand

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how gravity works. If that were
the case, and it was an inherent

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property of gravity that causes
galaxies to move the way they

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do, then we could expect all
galaxies to behave the same way.

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In other words, they would all
seem to have about the same

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portion of “dark matter.” But
with the galaxy in this Hubble

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image, astronomers looked at the
velocities of ten globular

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clusters in the galaxy, each a
spherical collection of hundreds

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of thousands of stars, and
calculated that their movements

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can be accounted for entirely by
the mass of the visible material

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in this system. That means this
galaxy has little to no dark

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matter. Strangely, this absence
of dark matter actually provides

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evidence that dark matter is
real. It shows that dark matter

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isn’t always coupled with
regular matter – that it’s

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something separate. You can have
regular matter without dark

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matter. This galaxy is really
weird even beyond the dark

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matter thing. You may have
noticed you can see straight

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through it. That’s because this
galaxy is what’s called an

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“ultra diffuse galaxy,” which as
the name implies, is extremely

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low density. This galaxy is
about the same volume as our own

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Milky Way galaxy, but only has
about 0.5% the amount of stars.

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Though astronomers have known
about ultra diffuse galaxies

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since the early 1980s, they can
be difficult to find since

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they’re so faint. A team of
astronomers is using an array of

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telephoto lenses called
Dragonfly to seek out these

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ghostly-looking objects. They
obtained observations from

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Dragonfly, the Sloan Digital Sky
Survey, the Gemini Observatory,

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and the Keck Observatory, then
requested time on the Hubble

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Space Telescope to take a closer
look at this unusual galaxy.

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Having images and data from
multiple sources allowed the

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team to determine that this
galaxy does not have a

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significant amount of dark
matter. This was definitely

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surprising to find. No other
galaxies so far have appeared to

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be so lacking in dark matter. In
fact, other ultra diffuse

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galaxies seem to have an
overabundance of dark matter.

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The same team who studied this
galaxy discovered a different

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ultra diffuse galaxy in 2016
that they calculated was 99.9%

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dark matter. Yet another weird
thing about this galaxy – the

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globular clusters used to
measure the galaxy’s rotation

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are way brighter than normal
globular clusters. The

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researchers have written a
different paper that focuses on

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just these oddball collections
of stars. So, this is a very

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strange galaxy in several ways.
Astronomers will be looking at

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Hubble observations of other
ultra diffuse galaxies to see if

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there are any other examples of
galaxies with unusually low or

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high amounts of dark matter.
With more samples, astronomers

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will be able to better
understand the nature of dark

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matter, the formation and
evolution of galaxies, and the

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overall structure of our
universe.

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www.nasa.gov/hubble
@NASAHubble