Galaxies are huge collections of stars, gas, dust, and dark matter that make up our universe. Scientists have learned many surprising things about them that change how we understand space.
Here are 15 discoveries that show just how big and amazing galaxies really are.
1. The Milky Way Weighs Around 1.5 Trillion Times the Sun’s Mass
The Milky Way, our galaxy, is incredibly massive. It weighs about 1.5 trillion times as much as the Sun. (ref)
This weight comes from stars, planets, gas, dust, and an unseen substance called dark matter. The size and mass of the Milky Way are much larger than we can see with our eyes, making it a vast and powerful structure in space.
2. Exoplanets Change Our Understanding of the Universe
Exoplanets are planets that orbit stars outside our solar system. Scientists didn’t know if other stars had planets until about 30 years ago. Now, they believe almost every star has at least one planet. (ref)
Some exoplanets don’t orbit a star; they float in darkness through the galaxy. With NASA’s Kepler spacecraft and the Transiting Exoplanet Survey Satellite, we are discovering more of these distant worlds every year, expanding what we know about planets beyond our solar system.
3. Galaxies Are Made Up of Nearly 90% Dark Matter
Most of the matter in galaxies, including our Milky Way, is invisible. Over 80% of a galaxy is made up of something called dark matter. (ref)
Scientists can’t see dark matter directly, but they know it’s there because of the way it affects things like stars and galaxies. Dark matter is mysterious and doesn’t interact with light, which is why we can’t see it, but it’s what holds galaxies together.
4. More Black Holes Discovered in the Early Universe
With the help of NASA’s Hubble Space Telescope, scientists have found more black holes in the early universe than they thought existed. (ref)
Black holes are regions of space where gravity is so strong that nothing, not even light, can escape. Discovering more black holes from the early days of the universe helps us understand how galaxies and stars formed billions of years ago.
5. The Local Group Houses More Than 50 Galaxies
Our Milky Way is part of a neighborhood in space called the Local Group. This group is made up of around 85 galaxies, including the Andromeda Galaxy and many smaller ones. (ref)
These galaxies are relatively close to each other in the vastness of space, and they are all bound together by gravity. The Local Group is just one part of an even larger structure called a galaxy cluster.
6. Merging Galaxies Form Massive Mega-Galaxies
When two galaxies collide and merge, they form a single, much larger galaxy. These mergers can happen slowly over millions of years.
When they come together, the stars inside them usually don’t crash into each other because they are spread out, but the overall shape of the new galaxy changes. Merging galaxies can create massive “mega-galaxies” that are much bigger than either galaxy was before.
7. Galaxies Feature Expansive Stellar Streams
Galaxies have long, thin trails of stars called stellar streams. These streams form when smaller galaxies or star clusters are pulled apart by a larger galaxy’s gravity.
The stars in these streams stretch out into space, making the galaxy appear even larger. Stellar streams are evidence of how galaxies interact and grow by “eating” smaller objects over time. (ref)
8. Ultra-Diffuse Galaxies Are Smaller Than the Milky Way
Ultra-diffuse galaxies (UDGs) are spread out, making them look faint, but studies found that they are ten times smaller in extension than Milky Way-like galaxies.
UDGs also have far fewer stars. These galaxies are hard to spot because of how faint they are, but their size shows that galaxies can exist in many different forms. (ref)
9. Dark Energy Makes the Universe Expand Faster
The universe is getting bigger all the time, but it’s not just expanding—it’s expanding faster and faster. This acceleration is caused by dark energy and scientists found that it’s pushing galaxies apart at an increasing speed.
Scientists are divided about its explanations; some believe that it’s vacuum energy (ever-present background energy), an energy fluid or field that fills space (quintessence), a defect in the fabric of the universe, and isn’t something physical that we can discover. (ref)
This mysterious force plays a huge role in the future of the universe.
10. Galaxies Fed on Gas Just 400 to 600 Million Years after the Universe Began
Not long after the universe was born, galaxies were already forming and growing. They did this by “feeding” on gas that filled space.
This gas allowed galaxies to create new stars and expand. Even in the early universe, just 400 to 600 million years after it started, galaxies were busy growing and developing into the giant structures we see today. (ref)
11. Galaxies Drift Apart as the Universe Expands
Hubble’s Law suggests that the space between galaxies is getting larger. (ref) As space itself expands, galaxies drift apart over time.
The Milky Way is in a small cluster called the Local Group. The largest is the Andromeda galaxy, which is moving toward the Milky Way. In a few billion years, they will collide and merge into a giant galaxy. (ref)
12. Andromeda Extends Over 260,000 Light-Years Across
The Andromeda Galaxy, which is the closest large galaxy to our Milky Way, is enormous. It stretches about 260,000 light-years across.
This means that if you traveled at the speed of light, it would take 260,000 years to go from one end of Andromeda to the other.
It’s much larger than our own galaxy, the Milky Way.
13. Galaxies Are Linked by a Vast Cosmic Web
Galaxies are not scattered randomly in space. Instead, they are connected in a huge network called the cosmic web. This web is made up of long strands of galaxies, with large empty spaces in between. (ref)
The cosmic web helps us understand how galaxies are spread out and how they interact with one another across the universe.
14. The Virgo Supercluster Covers 110 Million Light-Years
The Virgo Supercluster is a huge collection of galaxies, including our own Milky Way. It spans an enormous distance of 110 million light-years across.(ref)
This means that if you traveled at the speed of light, it would take 110 million years to cross the supercluster. It’s one of the largest structures in the universe, with many galaxy clusters inside it.
15. Dwarf Galaxies Contribute to Galactic Growth
Dwarf galaxies are small galaxies that are found to contribute to the growth of larger galaxies, including the Milky Way. According to scientists, stars hold clues about where they were born. In a study, they found an unusual chemical composition of a star indicating it came from a dwarf galaxy that was broken apart by the Milky Way. (ref)
When this happens, the larger galaxy gains more stars, gas, and dark matter. Dwarf galaxies might be small, but they help shape the universe by feeding into bigger galaxies.
As scientists continue to explore and learn more, we will uncover even greater wonders. The universe is full of surprises, and understanding galaxies is just the beginning of our journey into the stars.
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Nancy Maffia
Nancy received a bachelor’s in biology from Elmira College and a master’s degree in horticulture and communications from the University of Kentucky. Worked in plant taxonomy at the University of Florida and the L. H. Bailey Hortorium at Cornell University, and wrote and edited gardening books at Rodale Press in Emmaus, PA. Her interests are plant identification, gardening, hiking, and reading.
